ci/cd - add temporary test environments

[ntno]

see https://github.com/ntno/mkdocs-demo for example
new s3 bucket for each PR, deploy doc site to temporary website, teardown bucket when PR is closed

[ntno]

related feature:

long-lived example environments implemented for the following examples using GitHub Actions and GitHub Pages:

example name	source	site URL	pages repo
hilightjs	./tests/examples/highlightjs	https://ntno.github.io/mkdocs-terminal-example-highlightjs	ntno/mkdocs-terminal-example-highlightjs
hilightjs-dark	./tests/examples/highlightjs-dark	https://ntno.github.io/mkdocs-terminal-example-highlightjs-dark	ntno/mkdocs-terminal-example-highlightjs-dark

see PRs:

• refactor example site publish workflow #172
• improve example site publish workflow #173

wip PRs:

• experimental / WIP - fix typo in workflow YAML #167
• experimental / WIP - fix copy/paste error #166
• experimental / WIP - patch GH Actions workflow #165
• experimental / WIP - deploy sub site to external repo #164