add integrations page for chat docs

[vladvelici]

Description

Part of https://ably.atlassian.net/browse/CHA-1145 as "Use your own database" guide will have to be too long if it has to include all this info, and we don't cover it elsewhere.

JIRA ticket for this: https://ably.atlassian.net/browse/CHA-1226

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch chat-add-integrations-page

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[AndyTWF]

One minor comment.

I think we should have a review from DevEd before merging :)

[vladvelici]

Hey @franrob-projects, I see you volunteered to review this, thanks!

I've finished all the changes @AndyTWF suggested and rebased the commits. It's ready for a final look-over 🙏