Add warnings for Content Invalidation creation and documentation

[tcfdev]

Closes: #5698

This is an aesthetic change to add information warnings related to Content Invalidation. Specifically, this is to notify a person using this feature that the Origin(s) for the Delivery Service should support IMS and other proper cache invalidation requirements as specified in RFC 7234, Section 4.3.2.

If an origin server were to be misconfigured and only return 200s, disregarding IMS headers, then a Content Invdalidation request may have unintended load consequences.

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Which Traffic Control components are affected by this PR?

• Documentation
• Traffic Portal

What is the best way to verify this PR?

Launch and run Traffic Ops and Traffic Portal. Manually verify there is a warning, of appropriate visual representation, when attempting to create a Content Invalidation job at:

host.domain.tld/#!/jobs/new endpoint

(Navigate Tools -> Invalidate Content -> More dropdown -> Create Invalidation Request)

host.domain.tld/#!/delivery-services/<id>/jobs/new endpoint

(Navigate Services -> Delivery Services -> Select a DS -> More dropdown -> Manage Invalidation Requests -> More downdown -> Create Invalidation Request)

Documentation for content invalidation was also updated to reference RFC 7234 in both content invalidation locations

Using Traffic Portal - Invalidate Content

Quick How-To - Content Invalidation

PR submission checklist

• This PR has tests - Aesthetic changes only
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[mattjackson220]

looks good to me! tests pass, tested locally and works as expected! docs build and look good! ill merge after a rebase