Document how to create mason-registries

[titaniumtraveler]

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Is your feature request related to a problem? Please describe.

Currently there is no documentation on how to setup your own registries.

Describe the solution you'd like

How to add custom registries to mason is referenced in :h mason-registries,
but there is no documentation on how to actually create them.

I found out how to do it from looking through the test cases
(specifically
https://github.com/williamboman/mason.nvim/blob/main/tests/minimal_init.vim
https://github.com/williamboman/mason.nvim/blob/main/tests/helpers/lua/dummy-registry/index.lua)

but it would be nice if there were actual documentation for it.

Describe potential alternatives you've considered

I guess keeping it undocumented, or maybe just linking to the examples to let people figure it out by themselves.

Additional context

No response