[Portal] Add a "documents" section

[db0company]

Edit the page "documents" with all our documents downloadable. They will not be available on the "site vitrine" anymore.

[db0company]

Edit the document page of the new portal to centralize all the documentation we have:

• LabEIP rendu (in the Doc repository)
• Google Documents (ongoing documents)
• GitHub readmes
• Auto-generated documentation from the code

[db0company]

I updated the Portal so it uses the README.md file of the Doc repository, that I updated as well.