docs: fix low-level request example and align readme with other SDKs

[sachaservan]

The low-level HTTP example targeted https://example.com, which the client rejects at runtime because requests must go to the attested enclave; it now uses tfclient.enclave. The release_tag comment claimed it is None for bundled verification, but bundles do populate it.

Also reorders the README to match the other Tinfoil SDKs and links the prompt cache section to the shared docs page instead of repeating it.

---

Summary by cubic

Restructures the README to match the other Tinfoil SDKs and fixes two incorrect examples.

• Corrects the low-level HTTP example to target tfclient.enclave instead of https://example.com, which the client rejects at runtime.
• Fixes the release_tag comment, which incorrectly claimed it is None for bundled verification.
• Links the prompt cache section to the shared docs page instead of repeating it in full.

^{Written for commit db0d85b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}