Chat docs: change order of subsections

[jffortier]

Background

• Re-ordering the sections of the Chat documentation page. Its more important to start with the Chat source and its dimensions than app bridge.

Before:

- ChatProps
- App Bridge for checkoout
- Getting started with App Bridge for checkout
- App Bridge’s global variable
- Chat source and query parameters
- Chat dimensions
- App Bridge’s CSS API
- Example

After:

- ChatProps
- App Bridge for checkoout
- Chat source and query parameters
- Chat dimensions
- Getting started with App Bridge for checkout
- App Bridge’s global variable
- App Bridge’s CSS API
- Example

• Adding the port.start() in one of the example.
• Simplify the CSS. Remove the min() from the example.

[github-actions[bot]]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset directory. If the changes are user-facing and should cause a version bump, run yarn changeset to track your changes and include them in the next release CHANGELOG. If you are making simple updates to repo configuration, examples, or documentation, you do not need to add a changeset.