Backport custom 403 and 404 pages in Caddy

[stasadev]

Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem?

• feat(webserver): explain webserver-level 403s, fixes #8595, fixes #7345 (#8641) [skip ci] ddev#8641
• feat(webserver): explain webserver-level 404s, fixes #8588 ddev#8590

Describe your solution

Try to backport the new custom 403 and 404 pages that we're going to have in DDEV v1.25.4+.

Describe alternatives

If it's not possible in Caddy, leave it as is.

Additional context

No response