Update documentation

[awvwgk]

• update installation instructions for using mamba by default

• only include building from source with meson in the README

  • update list of dependencies and link to websites of (build) dependencies

• point to installation guide on webpage for CMake and fpm support, closes instructions on how to install ninja, CMake, fpm, and other package build approaches. #89

• update usage example to avoid error when requesting gradient, closes Top-level README: Running simple-dftd3 from the CLI Instructions Fail #97

• include instructions for running sphinx-build

• include instructions for running ford, closes Top-level README Instructions for Building Docs #96

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.24%. Comparing base (3a50cbd) to head (5269886).
Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   48.63%   50.24%   +1.60%     
==========================================
  Files          34       24      -10     
  Lines        3707     2448    -1259     
  Branches     1518      987     -531     
==========================================
- Hits         1803     1230     -573     
+ Misses       1207      877     -330     
+ Partials      697      341     -356     

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