Add a header comment to devsite md files.

[hsubox76]

v9 (current) docs are generated from source code in this repo, and then published to devsite in google3. Occasionally people submit internal CLs to make changes to the generated google3 devsite files. Adding a header to warn them that those changes will be overwritten eventually, and permanent changes should be made in this repo.

A sample generated file by this workflow looks like this (the file is analytics.analytics.md):
https://gist.githubusercontent.com/hsubox76/3e6b95c4b85884716c7a1ac3c77957dc/raw/6864d70fa300fe9eb53187586ffc896e2655fd7a/analytics.analytics.md

[changeset-bot]

⚠️ No Changeset found

Latest commit: 31bc1ec

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[google-oss-bot]

Size Report ¹

Affected Products

• @firebase/firestore

  Type	Base (0b385f1)	Merge (7a3fc06)	Diff
  main	426 kB	427 kB	+261 B (+0.1%)

• firebase

  Type	Base (0b385f1)	Merge (7a3fc06)	Diff
  firebase-auth.js	411 kB	411 kB	-28 B (-0.0%)

Test Logs

• Base (0b385f1): https://github.com/firebase/firebase-js-sdk/actions/runs/1661607391
• Merge (7a3fc06): https://github.com/firebase/firebase-js-sdk/actions/runs/1683750919

1. https://storage.googleapis.com/firebase-sdk-metric-reports/6TdEKdruJu.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (0b385f1) and merge commit (7a3fc06).

Test Logs

• Base (0b385f1): https://github.com/firebase/firebase-js-sdk/actions/runs/1661607391
• Merge (7a3fc06): https://github.com/firebase/firebase-js-sdk/actions/runs/1683750919

1. https://storage.googleapis.com/firebase-sdk-metric-reports/LWd0RaFfFP.html

[egilmorez]

LG, thanks!