Add docs on environment variables and wasm-specific behaviour

[ianthomas23]

Add documentation on environment variables and wasm-specific behaviour such as blocking requests and use of CORS proxy server.

[ianthomas23]

Closes #157.

[ianthomas23]

New pages are in the new Appendix starting at https://git2cpp--166.org.readthedocs.build/en/166/env_vars.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.89%. Comparing base (f8ef848) to head (26aeda8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
- Coverage   88.55%   86.89%   -1.66%     
==========================================
  Files          62       62              
  Lines        2893     2893              
  Branches      361      360       -1     
==========================================
- Hits         2562     2514      -48     
- Misses        331      379      +48     

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