Improve SEO by adding sitemap to sphinx docs

[tonyyanga]

This PR adds sitemap.xml to sphinx docs using sphinx-sitemap package. Including a sitemap improves SEO for the documentation site (source: stackexchange).

Here is the generate sitemap file hosted on a mirror of the sphinx docs: https://trafilatura-epsilla.readthedocs.io/en/latest/sitemap.xml

Here is the corresponding link in existing docs: it reports HTTP 404.

[adbar]

Hi @tonyyanga, thanks! I don't have any CI/CD pipeline for the docs yet, I'll test and integrate the PR manually next week.

[codecov]

Codecov Report

Merging #435 (8fa6f45) into master (2c3b751) will not change coverage.
The diff coverage is n/a.

❗ Current head 8fa6f45 differs from pull request most recent head 41adf07. Consider uploading reports for the commit 41adf07 to get more accurate results

@@           Coverage Diff           @@
##           master     #435   +/-   ##
=======================================
  Coverage   96.82%   96.82%           
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  Files          22       22           
  Lines        3340     3340           
=======================================
  Hits         3234     3234           
  Misses        106      106           

[adbar]

I added a robots.txt file pointing to the sitemap, just in case it might bring something.