Move CNI docs to their own files

[olemarkus]

The networking docs were getting a bit unruly.

Moved them to their own files and made them a little bit homogenic. Also removed the calico v3 upgrade docs that describes the process from before etcd-manager.

[johngmyers]

upgrade_from_kops_1.6_to_1.7_calico_cidr_migration.md is linked to from docs\releases\1.7-NOTES.md, which is why I submitted #8372.

[olemarkus]

Yeah, I know I broke a link there. I don't think keeping the old release notes brings any value the way we maintain our docs. They offer no value now, or even worse, they can be very misleading. I had to actually create a calico cluster with an old version of kops and upgrade to see if those docs made sense at all.

[rifelpet]

this looks great, thanks! We'll have to make a decision on what to do about broken links in docs specific to kops versions like release notes. k/k seems to keep around all of their old changelogs including broken links (for example the Documentation link here) if that is of any influence.

/approve
/lgtm

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