docs: add NOTICE, CONTRIBUTING, SECURITY, CODE_OF_CONDUCT; reorganize Readme navigation

[yaroslavaristov]

What

Adds standard OSS project documentation that was missing:

• NOTICE — attribution file per Apache License 2.0 §4(d). Single
  copyright holder (Niall Douglas, 2015-2026) confirmed across
  include//src/ file headers; no bundled third-party code requiring
  separate attribution.
• CONTRIBUTING.md — fork/branch/PR workflow (based on develop, not
  master), build/test commands from Build.md/AGENTS.md, code style
  notes.
• SECURITY.md — points to GitHub's private vulnerability reporting
  instead of public issues, with a scope section tailored to this project
  (memory safety, TOCTOU/race-free guarantees, path handling).
• CODE_OF_CONDUCT.md — Contributor Covenant 2.1.

Readme.md changes

Also reorganized Readme.md navigation, since the current badge table was
hard to read (CI badges squeezed between text links in one row):

• Split into a badges/title block (shields.io style) + a short table of
  contents, since the document is long (~200 lines, ~10 sections).
• Added explicit ## headers to sections that only had implicit ones (e.g.
  "Why you might need LLFIO"), so the TOC has real anchors to link to.
• No content was removed or reworded — only structure/navigation.

This last part is more opinionated than the docs additions above, so happy
to split it into a separate PR or drop it entirely if you'd rather keep the
existing Readme structure — let me know.

Why

Wanted to contribute something useful after using LLFIO — these are the
standard files most active OSS repos have and this one didn't yet.

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