docs: Add doco for db changes

[boc-the-git]

What type of PR is this?

• documentation

What this PR does / why we need it:

Adds some documentation for how to go about DB changes.
I wrote this ages ago as part of #2930 which is on hold, so splitting it out separately to unblock the docs.

Which issue(s) this PR fixes:

None.

[Kuchenpirat]

#3437 added task py:migrate

Might be good to use that in the dev guide as it sets the appropriate flags automatically and we can extend that if needed in the future.

mealie/Taskfile.yml

Lines 150 to 154 in 21f1d46

	py:migrate:
	desc: generates a new migration file e.g. task py:migrate:generate "add new column"
	cmds:
	- poetry run alembic revision --autogenerate -m "{{ .CLI_ARGS }}"
	- task: py:format

[boc-the-git]

#3437 added task py:migrate

Might be good to use that in the dev guide as it sets the appropriate flags and we can extend that if needed in the future.

mealie/Taskfile.yml

Line 150 in 21f1d46

py:migrate:

Good spot. Updated.

[Kuchenpirat]

Looks Good 👍