Add documentation for swappable forms.

[valberg]

Copy of #105 to get in into the new docs.

[codecov]

Codecov Report

Merging #181 (9c5d293) into master (a51c5d9) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
- Coverage   64.64%   64.57%   -0.08%     
==========================================
  Files          17       17              
  Lines         478      477       -1     
==========================================
- Hits          309      308       -1     
  Misses        169      169              

Impacted Files	Coverage Δ
invitations/app_settings.py	92.72% <ø> (-0.13%)	⬇️
invitations/utils.py	76.92% <ø> (ø)

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