Improve documentation of needed ports

[DL6ER]

Multi-purpose PR to improve the documentation about the necessary ports for Pi-hole:

• FTL only uses port 4711 for its telnet API (for quite some time)
• Add open resolver warning below the table of necessary ports to remind users that this table does NOT (as in: NEVER) read this table as a list of ports to open from the WWW onto their Pi-holes. This is just bad idea. Fullstop.
• Add a section about why we use random UDP ports by default (can be disabled, but we don't say this explicitly here)

This PR is independent of the FTL version and be merged right away.

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[DL6ER]

I could need some help with the broken tests here, I did not touch any of these links:

Furthermore, I checked for the first link in here and this site seems to be linked to from no page at all expect the navigation menu:

$ grep "github/contri" -Ri *
mkdocs.yml:    - 'Contributing on GitHub': guides/github/contributing.md

So why is this error even there?

[PromoFaux]

There are merge conflicts here in package.json, else LGTM

[DL6ER]

Rebased on latest master

[DL6ER]

Converting again to draft so we can update the recommendation from OpenVPN to Wireguard once the other PR is merged.

[DL6ER]

Rebased on latest master and changed VPN recommendation from OpenVPN to Wireguard

[DL6ER]

This PR is considered stale.