chore: update the README

[joeldierkes]

Note

Rewrites and expands the README with a clearer structure, new command reference and web search docs, benchmarks, updated configuration (including new env var defaults), and a how-it-works overview.

• Docs (README):
  • Restructure: Reorders content into Quick Start → Why mgrep → When to use what → Benchmarks.
  • Command Reference: Adds detailed mgrep search and mgrep watch options with examples; introduces a Web Search subsection under commands.
  • Configuration: Consolidates config file, environment variables, and tips; updates defaults for MGREP_MAX_FILE_SIZE (1MB) and MGREP_MAX_FILE_COUNT (1000); fixes links to point to Configuration.
  • How It Works: Adds internal overview of stores, reranking, and results metadata.
  • Agents & Extras: Clarifies agent install commands; adds Testing, Troubleshooting, Support; minor copy/format cleanups.

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