build: Markdown linter flags contents of `.sphinx` dir

[medubelko]

Situation

The Markdown linter isn't ignoring the contents of .sphinx, creating false flags for users and our PR builds fail:

$ make lint-md
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/container/README.md:32:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 2; Above] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/container/README.md:37:1: MD014: Dollar signs used before commands without showing output (commands-show-output)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/container/README.md:42:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 2; Above] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/container/README.md:59:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 2; Above] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/deflist/README.md:14:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 2; Above] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/deflist/README.md:36:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 2; Above] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:9:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:47:1: MD031: Fenced code blocks should be surrounded by blank lines (blanks-around-fences)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:49:1: MD031: Fenced code blocks should be surrounded by blank lines (blanks-around-fences)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:51:1: MD031: Fenced code blocks should be surrounded by blank lines (blanks-around-fences)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:59:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:60:1: MD031: Fenced code blocks should be surrounded by blank lines (blanks-around-fences)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:82:1: MD031: Fenced code blocks should be surrounded by blank lines (blanks-around-fences)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:83:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Above] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:105:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:108:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:111:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:114:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:118:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:121:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:124:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:127:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
/dev/sphinx-docs-starter-pack/docs/.sphinx/venv/lib/python3.12/site-packages/mdit_py_plugins/texmath/README.md:130:1: MD022: Headings should be surrounded by blank lines. [Expected: 1; Actual: 0; Below] (blanks-around-headings,blanks-around-headers)
make: *** [Makefile:111: lint-md] Error 1

Request

Fix the blocklist in the lint-md target.