Add a section with distro-specific considerations

[bmcustodio]

Over time we've been accumulating some knowledge about particular
Linux distributions and groups of distributions that has gone largely
unnoted in our documentation. A good understanding and implementation
of these considerations are extremely important to ensure that Cilium
runs properly, so this commit attempts at adding a subsection containing
this information.

[bmcustodio]

Adding @gandro (for Flatcar) and @borkmann (for new systemd notes) as reviewers as we've discussed these in some form over time.

[borkmann]

🚀

[gandro]

Looks good, thank you! The section likely applies to any distro that's using systemd-networkd as the default, but Flatcar is the only one we've observed this so far I think.

[aanm]

Instead of documenting, should we instead do a PR similar to #20072?