Refactor protected paths UI and documentation

What does this MR do and why?

Contributes to: technical-writing#452 (closed).

Improves the UI and docs for protected paths.

Closes: #330225 (closed).

Docs changes:

• Instance limits (before): https://docs.gitlab.com/ee/administration/instance_limits.html
• Instance limits (after): http://main-ee-72157.178.62.207.141.nip.io/ee/administration/instance_limits.html
• Protected paths (before): https://docs.gitlab.com/ee/user/admin_area/settings/protected_paths.html
• Protected paths (after): http://main-ee-72157.178.62.207.141.nip.io/ee/user/admin_area/settings/protected_paths.html
• Rack attack initializer (before): https://docs.gitlab.com/ee/security/rack_attack.html#rack-attack-initializer
• Rack attack initializer (after): http://main-ee-72157.178.62.207.141.nip.io/ee/security/rack_attack.html
• Rate limits (before): https://docs.gitlab.com/ee/security/rate_limits.html
• Rate limits (after): http://main-ee-72157.178.62.207.141.nip.io/ee/security/rate_limits.html

Screenshots or screen recordings

• Before:

  

• After:

  

MR acceptance checklist

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