Add timeline component and thread-history pattern

[jim-benson]

Task: task-1013232: Add timeline to atlas

Link: preview

This PR aims to add a vertical timeline component that uses the updated badge component and well as introduces the thread history pattern that is used in Q&A.

The styles and structure for the timeline component will be broad enough to be used is other instances of Learn.

Testing

1. git checkout jamesbe/thread-history; npm start;
2. Navigate to the timeline component
   Expected result: timelines usage should be clear
3. Navigate to the thread history pattern
   Expected result: the thread history pattern should be understandable to implement

Contributor checklist

• Did you update the description of this pull request with a review link and test steps?
• Did you submit a changeset? Changesets are required for all code changes.
• Does your pull request implement complex UI logic (js/ts)? Consider adding an integration test to test your user flow.
• Does your pull request add a new page to the documentation site? Add your new page for automated accessibility testing in /integration/tests/accessibility.

[changeset-bot]

🦋 Changeset detected

Latest commit: 7189d64

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@microsoft/atlas-site	Minor
@microsoft/atlas-css	Minor

Not sure what this means? Click here to learn what changesets are.

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