[docs] Describe how to block IPs using a firewall

[daenney]

Description

This adds some documentation on how to block one, multiple or whole IP ranges using the firewall. This can be helpful to protect from DDoS attacks or block certain parties from being able to communicate with your instance at all.

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.

[daenney]

https://gotosocial--2459.org.readthedocs.build/en/2459/advanced/security/firewall/#ip-blocking