chore(docs): add workspace watching feature doc

[fahslaj]

Description

Adds a Workspace Watching feature overview with examples to the lerna.js.org website.

Motivation and Context

This helps promote discovery of the new Workspace Watching feature and makes api reference more accessible.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (change that has absolutely no effect on users)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[isaacplmann]

Looks good. I just wanted to add one example. The github UI doesn't like the "```" in the suggestion so you'll have to fix that up in the real commit.