docs: Add FAQ for build failures, dim mismatch, and MCP install

[jose-compu]

Why

First-run failures usually cluster around CMake build, embedding dimension mismatch, and MCP binary discovery.

Task

Add a short FAQ to README.md (or docs/FAQ.md linked from README) covering:

1. Clean CMake rebuild tips
2. What happens / how to fix dimension mismatches on Put/Search
3. Installing / verifying logosdb-mcp-server

Acceptance

• Answers match current behavior
• Linked from README

Difficulty

Beginner — documentation only.