Add subproject SSC representative term length to documentation

[isabela-pf]

Fixes #138

As the Jupyter Accessibility council voted to have a one-year term length for our SSC representative, the result needs to be represented somewhere. I am not aware of any preexisting governance documentation for this project, so I created a page for this purpose. This PR includes

• A new governance page in governance.md that briefly outlines our subproject status and the term length we voted on.
• An amendment to the Community section index.md's note that references our status as a subproject. This note now points to governance.md

I'll admit I'm not very familiar with Jupyter Book and the docs structure we have now, so I expect to need to update this PR as I find errors. If you catch them before I do, feel free to point them out along with any other review. Thank you in advance!

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📚 Documentation preview 📚: https://jupyter-accessibility--140.org.readthedocs.build/en/140/

[gabalafou]

Thanks for updating the docs!

I only caught one small error. Apart from that, I think this good to go.

[gabalafou]

Oops I spoke too soon. A link needs to be fixed in index.md. Hold on... :)

[gabalafou]

I checked the RTD build. The link works now :)