Design proposal: Starter Prompts

[compulim]

Fixes #

Changelog Entry

Description

Adding design doc of the "Starter Prompts" feature.

Design

Starter prompts helps end-users to jumpstart a conversation by providing what the bot could help and a list of canned message that end-user can try out.

Specific Changes

• Added docs/STARTER_PROMPTS.md

• [x] ~I have added tests and executed them locally~
• [x] ~I have updated CHANGELOG.md~
• [x] I have updated documentation

Review Checklist

This section is for contributors to review your work.

• [x] ~Accessibility reviewed (tab order, content readability, alt text, color contrast)~
• [x] ~Browser and platform compatibilities reviewed~
• [x] ~CSS styles reviewed (minimal rules, no z-index)~
• [x] Documents reviewed (docs, samples, live demo)
• [x] ~Internationalization reviewed (strings, unit formatting)~
• [x] ~package.json and package-lock.json reviewed~
• [x] ~Security reviewed (no data URIs, check for nonce leak)~
• [x] ~Tests reviewed (coverage, legitimacy)~