Add an "edit this page" button to the docs

[mgoin]

Summary

It's useful to be able to immediately get to the source of a page on the docs to contribute edits, so this PR aims to add this as a button on each page.

Test Plan

Will test in the PR build

[zanieb]

Thanks for contributing! You can test the documentation locally with uvx --with-requirements docs/requirements.txt -- mkdocs serve -f mkdocs.public.yml

Looks like this renders as

The branch name is wrong though

I'm not entirely sure we want this, but an open to discussion. cc @charliermarsh

[mgoin]

Thanks so much for the prompt testing instructions! With the addition of edit_uri: blob/main/docs/, it seems to redirect to the right place

Clicking on the edit button on this page takes you to https://github.com/astral-sh/uv/blob/main/docs/index.md