create sitemap.xml

[orbeckst]

• install https://github.com/jdillard/sphinx-sitemap
• see create sitemap.xml for subprojects MDAnalysis.github.io#78
• fix create sitemap.xml for docs #20

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 4.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   76.68%   80.98%   +4.29%     
==========================================
  Files           2        2              
  Lines         163      163              
==========================================
+ Hits          125      132       +7     
+ Misses         38       31       -7

Impacted Files	Coverage Δ
pytng/pytng.pyx	80.62% <0%> (+4.37%)	⬆️

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