Rearrange Contributing pages + add gitpod info to contributing area

[reshamas]

Towards #178

• move Event Info (PR section) into "Contributing" section
• add Gitpod info to "Contributing" section

:books: Documentation preview en :books:: https://pymc-data-umbrella--216.org.readthedocs.build/en/216/

:books: Documentation preview es :books:: https://pymc-data-umbrella-es--216.org.readthedocs.build/es/216/

:books: Documentation preview pt :books:: https://pymc-data-umbrella-pt--216.org.readthedocs.build/pt/216/

[reshamas]

@OriolAbril Hopefully I did not make too much of a mess. The structure is what I am looking for.

I hope moving files around does not interfere with the translations. Not sure what the impact is.

[OriolAbril]

I am not sure about effect to translations. If it doesn't catch the rename and loses the info I'll rename the po files to keep that info