Add local dev docs for django-linear-migrations

[rossgray]

Changes

I came across the awesome django-linear-migrations tool we added recently but didn't see it in our docs, which means many people might not realise it exists.

Checklist

• Words are spelled using American English
• Titles are in sentence case

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Name	Status	Preview	Updated (UTC)
posthog	✅ Ready (Inspect)	Visit Preview	Feb 14, 2025 3:57pm

[Twixes]

Thank you! Just one tweak: to "Concurrent migrations"

[Twixes]

The migrations themselves are not concurrent! It's just about us using non-table-locking Postgres DDL statements, enabled by the CONCURRENTLY keyword. Proposing a bit of clarification:

Suggested change

	### Concurrent migrations
	Typically the migration generated by Django will not need to be modified, however, if you're adding a new constraint, such as an index, then you need to update the migration to run concurrently, so that it doesn't create a lock on the table. (Don't worry, there is a check in our CI that will flag if this is the case)
	### Non-blocking migrations
	Typically a migration generated by Django will not need to be modified. However, if you're adding a new constraint or index, you must tweak the migration so that it doesn't dangerously lock the affected table. We prevent locking by using the `CONCURRENTLY` keyword in Postgres DDL statements. Don't worry about this too much, a check in our CI will flag necessary tweaks as needed!