docs: document configuration-level runtime.tag

[soustruh]

Why

The configuration-level runtime.tag mechanism was underdocumented:

• integrate/jobs/index.md mentioned it in one sentence ("The values can be specified in the component configuration") with no key path and no example.
• The runtime bullet in extend/docker-runner/index.md named no keys.
• The section opener listed only backend and parallelism, though tag is documented three lines below.

Result: tooling that answers from these docs reports the tag can only be set in the job request, and gives no config-level JSON shape.

What

• integrate/jobs/index.md: added a JSON example of the config-level runtime node (sibling of parameters), how to read the resolved tag (job tag field), and a note that a pinned config keeps running the old image until the key is removed. Added tag to the section opener.
• extend/docker-runner/index.md: runtime bullet now lists the keys (tag, backend, parallelism) and links to the runtime settings section.

[soustruh]

Superseded by #380 (same change, branch renamed martin/ → soustruh/, commit amended to a clean single commit).