Add "Edit on Github" to secondary sidebar

[TimMonko]

References and relevant issues

xref napari/napari-sphinx-theme#216

Description

This adds an "Edit this Page" button to the secondary (right) sidebar on our docs.
Minimal setup was done via here: https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/source-buttons.html

And edit_page_url_template points not to the edit URL but instead to blob. I noticed from pydantic sphinx theme that clicking through leads me to the "You need to fork this" and not a nice clean document. I think being met with the fork screen would be a WTF moment as a new contributor, so I'd prefer something more welcoming (and where you can actually see the code right away)

Also ran into #981 while developing

[brisvag]

Can it also be renamed to "propose an edit" or something like that? Might be less intimidating, as "edit on github" might sound like it's off-limits to the casual visitor.

[TimMonko]

Hmm, at first my counter argument was that this language is "standardized", but I think you are on to something.

To do this, it does require a customized html template. If I do this I will add it to napari-sphinx-theme, so all repos benefit. So, in that sense this PR can be merged and then the upstream napari-sphinx-theme will override it later.

[TimMonko]

We collectively decided at the docs meeting, that it should remain "Edit on Github" since that is a standard. @brisvag if you feel strongly you can open an issue in napari-sphinx-theme ❤️