Add admonition to MKDocs

[idiazroncero]

We are using admonitions on our documentations in order to raise awareness about errors, warnings, notes, etc.

I think we should have this included by default on our mkdocs config. It is bundled with Material MKdocs (but inactive by default). It is as easy as adding

markdown_extensions:
  - admonition

to the mkdocs.yml file.

[jorgetutor]

+1

[idiazroncero]

PR ready.