docs: verify and document the KAS_IMAGE_VERSION values for how-to/build

[coretl]

Target page

how-to/build

The page instructs users to set KAS_IMAGE_VERSION but defers to the release
notes for the correct value, with a verify note in the source.

Category: verify-then-write

What a human must do (~1 hour)

1. Confirm the current valid KAS_IMAGE_VERSION value(s) for PandABlocks 5.x.
2. Document what the variable selects (which build container image / tag).
3. If the value is fixed per major release, add a table mapping release →
   KAS_IMAGE_VERSION.
4. If it tracks kas.yml automatically, update the note to say so clearly.

Post findings as a comment or PR against how-to/build.md.

Source

docs/how-to/build.md — the :::{note} block under "Build steps → step 3".