Add Custom migration template into the documentation

[Holicz]

Q	A
Type	improvement
BC Break	no
Fixed issues	#1122

Summary

Added documentation for the custom template. Grammar was checked using grammarly.com but please be aware english is not my first language.

[greg0ire]

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[Holicz]

Thank you for your time and the review! I accepted all of the comments and verified my email on Github.

[greg0ire]

Thanks @Holicz !

[Holicz]

I see the documentation was deployed, but the table with all available placeholders is not rendered.

I used a simple table according to this documentation, should I rewrite it to the grid table syntax in another PR? I checked some other parts of the documentation and the grid table is rendered correctly.

[greg0ire]

Yes, please go ahead.