Documentation updates

[awvwgk]

• add reference to MLatom as user of this library
• update C API documentation with explicit API references

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.13%. Comparing base (3a50cbd) to head (f572877).
Report is 32 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   48.63%   46.13%   -2.50%     
==========================================
  Files          34       25       -9     
  Lines        3707     2874     -833     
  Branches     1518     1298     -220     
==========================================
- Hits         1803     1326     -477     
+ Misses       1207      970     -237     
+ Partials      697      578     -119     

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