Document how to configure indexes and foreign keys

[bajtos]

This is a follow-up task for the proposal outlined in #2712.

Add a new documentation page How-tos >> Configure indexes and foreign keys, provide a high-level overview of indexes, foreign keys and how to define them for LB4 models.

Explain that we don't have full support for this feature yet, link to GitHub issues tracking the work.

Show how to configure indexes and foreign keys at model level via legacy juggler settings, link to LB3 docs: https://loopback.io/doc/en/lb3/Model-definition-JSON-file.html#indexes. Offer this approach as a workaround until we implement full support. (See also #2723)

Acceptance criteria

• A new documentation page as described above

[stale]

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