docs: Add Session Watcher intro

[fredppm]

What problem is this solving?
Add a small description about Session Watcher and why to use it.

[vtex-io-ci-cd]

Hi! I'm VTEX IO CI/CD Bot and I'll be helping you to publish your app! 🤖

Please select which version do you want to release:

• Patch (backwards-compatible bug fixes)

• Minor (backwards-compatible functionality)

• Major (incompatible API changes)

And then you just need to merge your PR when you are ready! There is no need to create a release commit/tag.

• No thanks, I would rather do it manually 😞

[vtex-io-docs-bot]

Beep boop 🤖

Thank you so much for keeping our documentation up-to-date ❤️

[github-actions]

	Messages
📖	❤️ Thanks!
📖	🎉 PR additions = 9, PR deletions = 1

Generated by 🚫 dangerJS against ed402a1

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[vtex-io-ci-cd]

Your PR has been merged! App is being published. 🚀
Version 1.39.1 → 1.39.2

After the publishing process has been completed (check #vtex-io-releases) and doing A/B tests with the new version, you can deploy your release by running:

vtex deploy [email protected]

After that your app will be updated on all accounts.

For more information on the deployment process check the docs. 📖