Missing documentation for custom migration template

The possibility of setting `custom_template` is not referenced in the docs, only in the docs for [DoctrineMigrationsBundle](https://symfony.com/doc/current/bundles/DoctrineMigrationsBundle/index.html).

Also, I was able to find only [one article](https://medium.com/@kcmueller/create-a-custom-template-for-doctrine-migrations-24d2b0314376) on this matter which is for version 2.x and is not working anymore since `` was removed and `` was added.

I think it would be best to add a new chapter with this information + information about all possible tags + example of a template.

I could provide a PR if agreed this part of documentation should be added.

Start with the DoctrineMigrationsBundle documentation linked in the issue and locate the current migration-template documentation in this repository. Identify the current custom_template option and supported tags, then add a chapter with a working template example and verify it against the current version.

Written by the indexing model from the issue text.