mono-docs: Indicate when a class is BETA

mono-docs: Indicate when a class is BETA

Summary:
The mono generator adds a <remarks> section to all BETA classes with an explanatory text.
These tags are understood by DocFX and Monodevelop.
On top of this, this commit modifies the DocFX template to give the Remarks section a more
prominent place in the page.

Test Plan: Render docs, Beta classes should have a big blue box with a remark about the class being Beta and dangerous.

Reviewers: lauromoura, vitor.sousa

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9381