Rust: Source and sink doc / tidy up

[geoffw0]

Source and sink tidy up:

• first commit: add qldoc to FlowSource.qll and FlowSink.qll explaining what they are, what the related classes are, and how it all fits together.
• second commit: add a QuerySink concept, a thin wrapper on DataFlow::Node that all Rust query sinks should extend now. This replaces the incomplete and easily forgotten getAQuerySinkKind in Stats.qll as a mechanism for finding all query sinks, e.g. for metric reporting.
• third commit: add the weak sensitive data hashing sinks to QuerySink (they were missing from getAQuerySinkKind).

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[geoffw0]

DCA LGTM (some additional sinks are detected as a result of the third commit; performance is unchanged).

[hvitved]

This QL doc does not render well in VS code. Consider using a code block, e.g.

 * ```
 * data from `*.model.yml` or QL extensions of `FlowSink::Range`
 *   v
 * `FlowSink` (associated with a models-as-data `kind` string)
 *   v
 * `sinkNode` predicate | other QL defined sinks, for example using concepts
 *   v
 * various `Sink` classes for specific data flow configurations <- extending `QuerySink`
 * ```

[geoffw0]

Ah, I wasn't aware this was actually rendered anywhere (it's rendered when mousing over an import, possibly other places).

Done.

[hvitved]

same about rendering

[geoffw0]

Also done.

[geoffw0]

I've also added regular expression injection sinks to the sink metrics now, since that query was just merged.