doc: refresh Bitgesell README overview

[hermesbountyhunter]

Summary

• Replace README content that described the standalone secp256k1 library with a Bitgesell-specific feature overview.
• Keep the existing build quickstart, but point readers to the repository's platform build docs instead of secp256k1 module flags.
• Summarize the actual project components: Bitcoin Core-derived node/wallet, LevelDB, secp256k1, Qt, and test suites.

Verification

• Documentation-only change; no build required.
• Reviewed the updated README diff locally.

Refs #32

[MyTH-zyxeon]

Maintainers - this looks like a plausible small #81 improvement-program candidate in the "codebase cleaning / README updates" lane, assuming the new overview now matches the actual Bitgesell project rather than the upstream secp256k1 library framing.

What seems reviewer-friendly here:

• Scope is narrow and non-runtime: one README.md rewrite, no consensus, wallet, RPC, or build-logic changes.
• The old README sections being removed were largely describing secp256k1 internals/modules rather than the Bitgesell node/wallet project itself, so replacing that with project-specific features looks directionally correct.
• The issue body for #81 explicitly says README/comment cleanups can be useful, even if they should likely be paid at a lower tier than substantive test/CI/code fixes.

Suggested acceptance checks:

1. Confirm every retained feature bullet is true for the current repo state (Keccak hashing, SegWit-only policy, fee-burn summary, block-weight target, halving cadence, max supply).
2. Confirm the README still points contributors to the right build entrypoints under doc/ for supported platforms.
3. Confirm the trimmed "Built With" list does not accidentally imply that embedded secp256k1 internals are the primary product rather than a dependency/component.

If you consider this too documentation-only for the main #81 lane, a quick maintainer note on the expected payout tier or acceptance bar for README cleanups would help contributors avoid guessing on future small PRs.