docs: rewrite README as short entry point with Node 22-26 policy (#966)

[Grace-ope]

Summary

Closes #966.

Supersedes #1076 (conflicts were too complex for the GitHub web editor: root guide delete/modify vs master).

This branch is cut from an updated master and re-applies the docs consolidation cleanly.

Changes

• Root keeps only standard meta docs (README, CONTRIBUTING, SECURITY, CODE_OF_CONDUCT, LICENSE, CHANGELOG) plus normal project folders
• Former root Markdown guides live under docs-site/docs
• README rewritten as a short entry point: what it is, requirements, quick start, feature map, links
• One Node policy: 22–26 (>=22 <27) with pnpm 9+, aligned with package.json engines
• Removed stray closes #550 and the old long fee-prediction-only README shape

Out of scope

• Vercel team authorization (maintainer/team only)
• Unrelated SDK work ([2026 Stellar] Upgrade @stellar/stellar-sdk to the latest major and migrate SorobanRpc to the rpc namespace #970) unless already on this branch by design

Test plan

• Root no longer lists the moved guide files
• Guides open under docs-site/docs
• README Node range matches package.json engines
• CI / docs drift checks pass when workflows are approved

Note for maintainers

Please close #1076 as superseded by this PR if it is still open.

[vercel]

@Grace-ope is attempting to deploy a commit to the nanle-code's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Grace-ope]

Superseded by #966 — fresh branch from updated master to avoid conflicts that cannot be resolved in the web UI.

@Nanle-code