Documentation for pelicanconf.yaml is scattered around and incomplete

[sebbASF]

There does not appear to be a definitive document that describes the contents of pelicanconf.yaml.
There are several partial documents, but these are not complete:

• https://infra.apache.org/asf-pelican-config.html which references
• https://github.com/apache/template-site

Note that neither of them describe what the site/index entry is for.
Nor is the new postrun feature documented.

Given that the file is processed by buildsite.py in this repo, it would seem sensible to document the syntax here.
For example by creating a same yaml file with all the possible entries.
This could even be used in unit tests.