docs: document deno upgrade pr (2.8)

[lunadogbot]

Summary

Documents the new deno upgrade pr <number> subcommand shipping in Deno 2.8 (denoland/deno#33252). It downloads the binary built by CI for a given denoland/deno PR via the gh CLI and installs it, optionally to a path with --output.

• New "Install a build from a pull request" section in runtime/reference/cli/upgrade.md.
• Notes the gh CLI prerequisite, #-prefix tolerance, the --output/--dry-run interactions, and the {profile}-{os}-{arch}-deno artifact-naming scheme.

Test plan

• deno task serve — section renders.

[fibibot]

The new deno upgrade pr <number> section matches denoland/deno#33252: the examples use the accepted pr subcommand shape, --output and --dry-run are parsed before pr, and the gh/artifact description matches the implementation. No checks are reported on this head, so holding approval until CI is green.

• nit: last_modified: 2026-03-05 should be bumped for this content addition.
• nit: Numeric prefix is also accepted is a little off; this example is accepting a # prefix.

[fibibot]

Thanks for documenting this. The command behavior matches denoland/deno#33252 and the current cli/tools/upgrade.rs implementation, but I think this needs one docs-policy fix before merge:

• runtime/reference/cli/upgrade.md has last_modified: 2026-03-05 unchanged while this PR adds a substantive new section. Repo policy asks substantive content changes to bump last_modified, so please update it.
• While touching it, please also change the comment above deno upgrade pr '#12345' from "Numeric prefix is also accepted" to something like "A # prefix is also accepted". The parser strips #; it is not a numeric prefix.\n\nI did not find a factual issue with the deno upgrade pr behavior itself.