Docs: Expand FAQ for 2.0 launch

[clockwork-labs-bot]

Rewrites the FAQ from 5 basic questions to a comprehensive guide covering:

• General — What is SpacetimeDB, is it only for games, is it blockchain, licensing, self-hosting
• Comparisons — vs Mirror/Photon/networking libs, vs Firebase/Supabase, vs PostgreSQL
• Architecture — Data persistence, real-time sync, reducers vs REST, module languages, views, procedures
• Development — Getting started, spacetime dev, SQL usage, Unity/Unreal/React/framework support, auth
• Deployment — Production publishing, hot-swapping modules, schema migrations, size limits, pricing
• Troubleshooting — Global database names (401/403), confusing generate errors, resetting databases

Content drawn from README, architecture docs, Zen of SpacetimeDB, and common user questions from GitHub issues and Discord.

[cloutiertyler]

Left some comments, please make sure to address all the questions listed here:

• How do I implement auth in my app?
  • Can I use Identity as a 1-to-1 with users? (We'd like you to, yes. Use this as an opportunity to talk about OIDC. Link to an OIDC-compliant provider users can use, ideally.)
  • Can I store passwords (or hashes thereof) in a private table, and have users log in by calling a reducer? (No, we don't have private reducer arguments, so any subscriber can see your password.)
  • How do I create a new localhost identity? (POST /identity.)
• How do I hotswap-publish? (It's easy.)
• How do I clear my database? (With -c.)
• How do I do migrations?
  • ...Add a column to a table? (Can't RN; redirect to "incremental migrations" for now.)
  • ...Remove a column from a table? (Same as above.)
  • ...Add a new table? (You can just do that.)
  • ...Add or remove indices? (You can just do that, though removing indices may break subscription queries.)
• How do I build a room- or match-based game? (You need an external service which creates and destroys StDB databases.)
• I updated SpacetimeDB (from a pre-1.0 version) and now I can't publish! (Delete your local data.)
• I got a weird error when I tried to publish! (Make sure you're using the same version of the module library as your host.)
• I got a weird error when I tried to compile my client! (Make sure you're using the same version of the client SDK as your spacetime generate codegen.)
• How do I subscribe to things related to a specific Identity? (Write a query that filters with 0x{identity_hex}.)