Restructure continuous scanning

What does this MR do?

Restructure documentation of continuous scanning.

Briefly, the proposal is to have a page that describes continuous vulnerability scanning. This page is to describe methods by which security scanning is triggered, other than manually (in a CI/CD pipeline and by policy). As it is, only one such method exists, that used by CCS and CDS. That page will then link to the individual pages for CCS and CDS.

In bullet-point form:

• Create new CVS page, to describe the what, why, and how, of CVS.
• Split the current detailed CVS page into 2 separate pages - one for CCS and one for CDS.
• Mention CVS in the "Detect" page, which describes the various ways in which security scans are triggered.

Note: I'm still a little uncertain about splitting the existing CVS content into 2 separate pages, because it's likely to be almost identical. However, I'll go ahead and try it and see if it works.

Related issues

[meta] Feature name: Continuous Vulnerability S... (#477593)

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