📖 Improve NuGet readme and document doc protection tag

[pleonex]

Small fixes after testing for the first time to deploy NuGets:

• By default GitHub pages are not allowed to publish from tag builds. Documented how to allow it.
• Improve README in NuGets by removing HTML in markdown.
• Removed trigger on release published as it triggers the build twice. We have already the trigger for git tag pushes.

Quality check list

• Related code has been tested automatically or manually
• Related documentation is updated
• I acknowledge I have read and filled this checklist and accept the
  developer certificate of origin

Acceptance criteria

• README in NuGet does not contain HTML or images
• Document how to publish docs from git tag builds

Follow-up work

None

Example

No need.