Docs: fix the port 53 and OpenPLi guides

[jackspiering]

Description

Three guides in documentation/ fixed.

• Free up port 53: the guide disabled the stub listener but left /etc/resolv.conf pointing at it, so host DNS stopped working. It now relinks /etc/resolv.conf before the restart, checks with getent hosts github.com, includes a rollback, and mentions 127.0.0.54.
• Tailscale on OpenPLi: the heredoc is quoted, so the init script no longer expands its own variables when it is written. ---retry is now --retry=TERM/9/KILL/11, $PIDFILE and $DAEMON are quoted, and the pinned 1.86.2 is replaced by a pointer to the static builds. The guide is retitled "Install Tailscale on an OpenPLi set-top box (without Docker)".
• Standard setup: corrects TS_AUTHKEY, ./config, and the "joins as a new device" sentence, and adds a note about required secrets.

Related Issues

• None.

Verification

• Extracted the init script from the edited guide, ran it with /tmp paths and a stand-in daemon: start, status, stop, restart and the usage message behave correctly. shellcheck reports one intended SC2086 on $DAEMON_OPTS.
• Confirmed the archive naming (tailscale_<version>_<arch>.tgz) on pkgs.tailscale.com.
• Confirmed with a throwaway stack that docker compose up overwrites ./config/serve.json from the configs content.
• rumdl check --config .markdownlint.yml . (0.2.78) and git diff --check are clean.
• Not run: the port 53 steps (this host has no systemd-resolved; they follow the systemd man page and Pi-hole's docs), and the OpenPLi guide on real hardware.

Checklist

• I have performed a self-review of my code and followed the templates structure.
• I have added verification that the stack works as expected.
• I have updated necessary documentation (e.g. frontpage README.md ).
• I have selected the correct label(s) for this PR.

Additional Context

From an external review of the repository (section 3.2). The root README links the OpenPLi guide under its new title in the README PR.