Add configurations to show the 'Edit on GitHub' button

[seisman]

Currently, each page on the documentation shows a "View page source" link, which links to the plain text source file:

It's more useful to link directly to the source files in the GitHub repository, because the source files are rendered well on GitHub and are easier to read. It's also much easier for users to submit quick fixes for typos.

The html_context is documented in readthedocs/sphinx_rtd_theme#465.

It works well in my other projects, so I expect it should work well for mspass too. However, I can't test it because for me, it's difficult to build mspass from source codes locally.

[codecov-commenter]

Codecov Report

Merging #222 (70c08f6) into master (82d6582) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
- Coverage   43.22%   43.21%   -0.01%     
==========================================
  Files         117      117              
  Lines       14899    14899              
==========================================
- Hits         6440     6439       -1     
- Misses       8459     8460       +1     

Impacted Files	Coverage Δ
python/mspasspy/db/database.py	82.04% <0.00%> (-0.08%)	⬇️

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[wangyinz]

Great! This is what I have thought about but did not have the chance to try it out. Thanks for contributing!