Add docs page about the meta.yaml specification

[agriyakhetarpal]

"The meta.yaml specification" page but I couldn't find it. As it turns out, it's gone with pyodide/pyodide#6181 and we never added that page to pyodide-build's docs. We should add it at some point.

Originally posted by @agriyakhetarpal in #412 (review)

[ryanking13]

I intentionally excluded meta.yaml spec from the pyodide-build docs, as our opinionated way to build packages is now using the pyodide build CLI or using cibuildwheel. I personally do not want any people (or AI agent) except the three of us (who are maintaining pyodide-recipes) to learn about the meta.yaml spec anymore.