Update agent instructions

[eerhardt]

No reason to run pnpm run build after making doc changes as it takes too long.

[Copilot]

Pull request overview

Updates contributor/agent guidance to streamline how documentation changes are validated in this repo.

Changes:

• Adds a new “Verifying documentation changes” section to AGENTS.md.
• Recommends using pnpm dev instead of a full pnpm run build for doc-only verification.

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[Copilot]

The guidance here is a bit misleading because pnpm run build (via src/frontend's build script) also runs pnpm linkcheck before astro build. For documentation changes—especially link updates—skipping build means you may miss broken internal links that only get validated during linkcheck/production build. Consider rewording to say pnpm dev is enough to verify rendering, but pnpm linkcheck/pnpm build(:production) should be run when changing links/routing/sidebar content or before merge when feasible (or rely on CI for that).

Suggested change

	After making documentation changes, running `pnpm dev` (the dev server) is sufficient to verify that the changes render correctly. A full `pnpm run build` is not required because it takes a long time.
	After making documentation changes, running `pnpm dev` (the dev server) is sufficient to verify that the changes render correctly. However, because `pnpm run build` also runs `pnpm linkcheck`, you should run `pnpm linkcheck` or `pnpm run build` when you change links, routing, or sidebar content, or before merging when feasible (or rely on CI to run these checks).

[IEvangelist]

No, pnpm linkcheck does a full build.