user: Bootstrap loaders high-level documentation
Add landing page of loaders high-level documentation displaying available ones in a table with the following column:
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logo and name of the loader linking to its specific documentation
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related links: source code, developer doc, development activity
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current status: production, staging or development
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link to related grant if any
Create one rst file per loader to further document.
Related to #3117
Migrated from D7637 (view on Phabricator)