Add docs for libmdanalysis

[hmacdope]

Fixes #3921

Changes made in this Pull Request:

• Adds a documentation section for libmdanalysis

PR Checklist

• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[codecov]

Codecov Report

Base: 94.36% // Head: 94.36% // No change to project coverage 👍

Coverage data is based on head (de3e18b) compared to base (fc9dee0).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3922   +/-   ##
========================================
  Coverage    94.36%   94.36%           
========================================
  Files          194      194           
  Lines        25109    25109           
  Branches      3398     3398           
========================================
  Hits         23694    23694           
  Misses        1366     1366           
  Partials        49       49           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[orbeckst]

Could you show a simple example for how the header is imported and how something in it is used (e.g., instantiate a Timestep)?

Is there a way to generate a list of publicly defined objects? If we cannot generate such a list automatically, could you mention the most important ones (i.e., Timestep at the moment as far as I can tell) and then say something along the lines "for details consult the source file" (and perhaps link to MDAnalysis/lib/libmdanalysis/init.pxd.

[orbeckst]

lgtm, just making two changes