Create requirements.txt

[j08lue]

Wanted to build the docs and found out I needed these packages for it.

Would it make sense to add a little requirements.txt here?

• [x] Added docs requirements.txt
• [x] Changed CI to use that file https://github.com/developmentseed/eoAPI/blob/458c185f9aceb86057bbb91b56c7e143f2282c40/.github/workflows/deploy_mkdocs.yml#L31