user: Bootstrap loaders high-level documentation

Add landing page of loaders high-level documentation displaying available ones in a table with the following column:

• logo and name of the loader linking to its specific documentation

• related links: source code, developer doc, development activity

• current status: production, staging or development

• link to related grant if any

Create one rst file per loader to further document.

Related to #3117

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Migrated from D7637 (view on Phabricator)