Document new indexes creation in 2021 HF

We need a document section to describe how MongoDB indexes should be managed on existing instances.
Since LTS 2021 indexes are defined in document schemas and created during Nuxeo start.
This is fine when starting from scratch, but is not recommended on existing instance with large amount of documents,
because creating an index is an heavy operation that can timeout or impact the MongoDB performance.

The recommended solution for large instance should be:

• disable the auto create index on start
• read carefully the upgrade notes when applying HF, they should contain commands to create manually the new indexes.