Improve documentation about custom templates.

[AlbertoGP]

Now includes all the template hooks in the page, and a detailed
example for the common case of adding an "about" page.

As requested in the comments at "#2115 Custom Routing / new Pages?!"

[codecov-io]

Codecov Report

Merging #3345 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3345      +/-   ##
==========================================
- Coverage   35.04%   35.03%   -0.02%     
==========================================
  Files         280      280              
  Lines       40571    40571              
==========================================
- Hits        14219    14214       -5     
- Misses      24248    24251       +3     
- Partials     2104     2106       +2

Impacted Files	Coverage Δ
models/repo_indexer.go	52.91% <0%> (-0.98%)	⬇️
models/repo.go	43.19% <0%> (-0.19%)	⬇️

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[lunny]

LGTM