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]

Please associate your email address with your Github account, or change the
email in your commits to an address already associated with it. If you do not
want to expose your personal email address, you may use
[email protected], that way we can still reach you through Github.
Additionally, you will get credit for this contribution on your Github profile.

[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.