docs: Caddyfile config adjustments

[francislavoie]

Some wording adjustments and correctness fixes:

• "directives" are specifically things within site blocks. Global options are not directives.
• * is still needed for the root directive for now
• the worker script example was missing php_server which was misleading because it wouldn't do anything without it.
• technically, the php_server long-form includes a route wrapper which guarantees the directive order stays as written.

[francislavoie]

Build Linux x86_64 binary
Username and password required

Looks like the CI config needs to be updated to avoid having to connect to Docker Hub.

[dunglas]

Thanks Francis!