docs(public-docsite-v9): add Nightly Releases concept page

[Copilot]

Fluent UI v9 publishes nightly releases on a workday cadence, but the mechanism, version scheme, and recommended consumption pattern are undocumented. This adds a single concept page covering both, so partners can wire nightlies into CI and catch regressions before they reach a stable release.

Investigation (grounding for the doc, not invented)

• Publish pipeline: azure-pipelines.release-vnext-nightly.yml, cron 0 4 * * 1-5 from master.
• Version bump: workspace-plugin:version-bump --bumpType nightly --prereleaseTag "nightly-<YYYYMMDD>-<HHMM>" → 0.0.0-nightly-<YYYYMMDD>-<HHMM>.<N>.
• Publish: beachball ... --tag nightly → npm dist-tag nightly.

Changes

• New page apps/public-docsite-v9/src/Concepts/NightlyReleases.mdx — uses the standard <Meta title="Concepts/..." /> convention and is auto-picked up by the ../src/**/*.mdx glob in .storybook/main.js. Sections:
  • Cadence, linked to the real ADO yaml.
  • Version/dist-tag identification with a npm view snippet.
  • Copy-pasteable scheduled GitHub Actions recipe that layers @fluentui/react-components@nightly on top of a stable yarn install, runs against the app's existing tests, and uses continue-on-error: true so it acts as an early-warning signal rather than blocking CI.
  • FAQ explaining the 0.0.0- prefix (keeps nightlies below any ^ range) and that nightlies are not semver-stable.
• Beachball change file: type: none for @fluentui/public-docsite-v9.

CI recipe (excerpt)

on:
  schedule:
    - cron: '0 8 * * 1-5' # a few hours after Fluent UI publishes
jobs:
  test-against-nightly:
    continue-on-error: true
    steps:
      - run: yarn install --frozen-lockfile
      - run: yarn add @fluentui/react-components@nightly
      - run: yarn build && yarn test

Notes for reviewers

• Owner requested: @microsoft/cxe-prg; PM sign-off likely wanted on the CI recipe wording.
• The recipe intentionally mixes @nightly (vNext packages) with @latest (e.g. @fluentui/react-icons, which is not part of the nightly pipeline); called out inline in the doc.