docs: explain how to trust the Caddy authority in Firefox

[ker0x]

Firefox uses its own trust store, so the commands in docs/tls.md (which update the system trust store) are not enough for it.

Also, when the caddy_data volume is removed (e.g. docker compose down -v), Caddy generates a new local authority with the same name as the previous one. If Firefox still trusts the old root, it rejects the new chain with SEC_ERROR_BAD_SIGNATURE instead of the usual SEC_ERROR_UNKNOWN_ISSUER (reported in #969 (comment)).

Changes

• Docs: add a Firefox section to docs/tls.md, explaining how to trust the authority (import root.crt, or enable security.enterprise_roots.enabled) and how to fix SEC_ERROR_BAD_SIGNATURE after the authority is regenerated.

Testing

Reproduced with Firefox (Playwright build) against FrankenPHP v1.13.0 / Caddy v2.11.7:

• fresh profile: SEC_ERROR_UNKNOWN_ISSUER
• profile trusting the current root: connection OK
• same profile after docker compose down -v && docker compose up --wait: certificate error other than SEC_ERROR_UNKNOWN_ISSUER, while the served chain still verifies with openssl verify against the new root.crt