Support "top_of_page_button": "blob" to avoid requiring sign-in

[umarcor]

Currently, the optional "pencil" button at the top of the page can only be set to either edit or None (see https://pradyunsg.me/furo/customisation/#top-of-page-button).

On GitHub, accessing the edit link requires being signed in. However, the equivalent blob URL can be browsed without signing in. See, for instance, https://github.com/pradyunsg/furo/edit/main/docs/quickstart.md and https://github.com/pradyunsg/furo/blob/main/docs/quickstart.md. Users who are already signed in can go from blob to edit through a single click. Hence, I typically prefer my docs to point to the blob URL.

I looked at the code and it seems non-trivial to change, since this is hardcoded in a dependecy:

• https://github.com/pradyunsg/furo/blob/main/src/furo/theme/furo/components/edit-this-page.html#L2
  • https://github.com/pradyunsg/sphinx-basic-ng/blob/main/src/sphinx_basic_ng/theme/basic-ng/components/edit-this-page.html#L30-L33

For completeness, it can be worked around through the custom edit button (see https://pradyunsg.me/furo/customisation/edit-button/#with-arbitrary-urls). However, AFAIAA the icon would remain the same.

I believe that making docs generally accessible to the general public is a legit use case to consider supporting this feature through source_repository, source_branch and source_directory. @pradyunsg would you be open to supporting this in sphinx-basig-ng?

Refs:

• https://sphinx-rtd-theme.readthedocs.io/en/stable/configuring.html#confval-vcs_pageview_mode
• Seeking feedback on new footer design! #374

[pradyunsg]

Same as #478