Add README.

[wangpatrick57]

Summary: Wrote Quickstart and Overview sections. Go here to see the README on the website.

Details:

• Overview summarizes the research motivation behind the project, giving background as necessary.
• Quickstart gives a single shell script which compiles Postgres with Boot, generates data, builds a Proto-X embedding, and trains a Proto-X agent.
• I renamed all occurrences of "pgdata" to "dbdata" to match the project's vision of working for multiple DBMSs (as described in the README).
• I removed the startup check.
• I got rid of the ssd_checker dependency as it's a very small repository.
• Fixed Postgres compilation code to work with the new vldb_2024 branch of Boot.