[docs] added split-domain configuration for Caddy 2

[wuchyi]

Description

Added a section for Caddy 2 with an example to set up a split-domain configuration, since I did not find this anywhere online when setting up my own GtS deployment.

(Also this is my first PR I've ever created on GitHub, so forgive me if it's not per standard operating procedures! Pointers and comments appreciated)

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[wuchyi]

Removed the lexer tag for caddy as it's not supported on Pygments, and I can't find a suitable replacement for it. Thanks @daenney!