Adding description to meta.yaml docs

[jakirkham]

Would be nice to add description to the meta.yaml docs with a brief description of what it is and how to use it. Guessing this is the right document.

[rrigdon]

Should go here: https://docs.conda.io/projects/conda-build/en/latest/resources/define-metadata.html

[github-actions[bot]]

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Thanks!

[jakirkham]

This hasn't been added, but still could be

https://docs.conda.io/projects/conda-build/en/latest/resources/define-metadata.html#about-section