docs: improve docs about pppoe and mark

[mzz2017]

Background

This PR doesn't really change many codes, but provides a solution for pppoe.

See #304.

Checklist

• The Pull Request has been fully tested
• There's an entry in the CHANGELOGS
• There is a user-facing docs PR against https://github.com/daeuniverse/dae

Full Changelogs

• Refine ebpf code.
• Improve docs about pppoe and mark.

Issue Reference

Closes #75

Test Result

[piyoki]

Will the changes made to the eBPF code implicitly affect non-pppoe users if they ONLY use the LAN interface?

[dae-prow]

❌ Your branch is currently out-of-sync to main. No worry, I will fix it for you.

[mzz2017]

Will the changes made to the eBPF code implicitly affect non-pppoe users if they ONLY use the LAN interface?

No. This PR doesn't introduce new code.

[mzz2017]

I plan to add two hooks to run in dae's lifecycle. This work should be done in the next PR.

[mzz2017]

I plan to add two hooks to run in dae's lifecycle. This work should be done in the next PR.

This issue can be done in systemd. Related way is appended in the discussion.

[piyoki]

Already tested, no abnormal behavior was detected.

[dae-prow]

🧪 Since the PR has been fully tested, please consider merging it.