docs: route SUPPORT.md at the two orientation threads, and stop claiming Q&A is empty

[laurentiu021]

Why

Seeding a Q&A thread made a sentence in SUPPORT.md false: "nobody has asked anything there yet". That is a live count written into prose, so it was going to expire the moment the thing it described succeeded. Reworded to something that cannot expire rather than patched with a new number.

What changed

• The Q&A row now reads "quiet enough that a question gets an answer rather than being buried" — the same reassurance, no count.
• A short paragraph at the top links the two new orientation threads: Start here (Start here — where to ask, and what to expect #2346) is the same routing as this file plus what to expect and how long it takes, and Roadmap (Roadmap — what is being worked towards, and what is not #2345) answers the three recurring ones — unsigned download, no telemetry, no Store build. Linking them from the docs means they route people whether or not they are pinned, which matters because pinning a discussion is a web-UI-only action with no API.
• The SmartScreen row gains the Q&A thread (Why does Windows warn "Windows protected your PC" the first time I run SysManager? #2347) beside the README anchor, for readers who would rather have it as a question and an answer than as a section.

Checks

• Grepped every .md for other emptiness claims about Discussions: none. The one remaining "nobody has" is about VirusTotal submissions and is still true.
• README's Q&A row makes no count claim, so it needed no change.
• Docs-only, docs: — no release, no version bump, no CHANGELOG entry, matching docs: correct the screenshot inventory and hold both counts against source #2339 and docs: name each screenshot after its tab instead of a position number #2342.

Refs #1665