[Hotfix] Cleanup README (+ RTD sync)

[yoonspark]

Summary

As per recent feedback from an external reviewer, we want to improve README so that a new user can onboard more quickly/effectively.

Changes

• [x] Use a simpler example for Quick Start section
• [x] Add instruction for default kernel (Python 3 (ipykernel))
• [x] Specify prerequisites for LineaPy installation
• [x] Fix RST syntax copied over to README
• [x] Put Interfaces section before Quick Start so the user knows how to load LineaPy extension before hands-on practice
• [x] Reduce amount of upfront info by removing non-essential contents from README (instead, direct them to RTD pages for more info)

PR also includes changes to sync RTD pages with the above README updates.

Testing

This PR does a pure docs update, so no need for automated testing. Examples used in docs have been validated locally.

Ticket(s)

This PR resolves https://linea.atlassian.net/browse/LIN-700.

[becca-miller]

I've made some minor edits to the README based on our call earlier today!

[yoonspark]

@dorx I am merging this PR for ODSC which is happening today.