SAST Docs: Point to analyzers instead of upstream projects

What does this MR do?

We've seen cases where people go to upstream projects for documentation or support. This is confusing for users and upstream projects, and reduces the visibility of things we spend effort maintaining, like CHANGELOGs for each analyzer. It also hides some of the value that GitLab provides, for example by maintaining a supported set of Semgrep rules.

Linking to analyzers instead of upstream makes it clearer that we are packaging and supporting analyzers, not using open-source projects directly off-the-shelf.

Adding "with GitLab-managed rules" to Semgrep rows in the table clarifies that Semgrep/r2c are not responsible for rules. We have seen rule issues reported upstream in some cases and wish to both clarify the value GitLab provides and reduce the effort Semgrep contributors spend redirecting to GitLab.

List of changes:

• Add language detection and scanning to the features-by-tier table
• Change links from upstream project websites/repos to GitLab analyzer projects
• Change "Semgrep" to "Semgrep with GitLab-managed rules"
• Change table column headers
• Format table whitespace/adjust alignment
• Remove mention of package managers, which aren't relevant to SAST—this may be an unintentional copy from Dependency Scanning
• Edit intro text for the language support table

Related issues

I can gather some but don't have them immediately at hand.

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