Restructure v2.6 Security section

[macedogm]

This is an initial work preparing for the updates to CIS hardening guides for v2.6 - rancher/rancher#35735.

1. Remove v2.5 related hardening guides from v2.6 and mentioned that v2.6 guides are being updated.
2. Added aliases for removed pages.
3. Update Kubernetes security best practices.
4. Fix some small typos.

[vincent99]

Removed or renamed files == broken links in search engines indefinitely. Setup redirects in frontmatter to preserve all existing paths and go to wherever is appropriate.

[macedogm]

@vincent99 thanks for catching this. Is this - https://github.com/rancher/docs/blob/master/nginx.conf - the place where I can set those redirects or you mean to just preserve the original _index.md of the affected page and set the redirect there?

[vincent99]

Nginx is easier for bulk pattern matching/folder renames, aliases for individual pages.

[macedogm]

@vincent99 thanks again for the tip. I added aliases for the removed pages. No links will be harmed now 😂