docs: polish the README

[owenthcarey]

Summary

Restructures README.md to be easier to scan and more polished, without changing what it says about WeaveC.

• Centred header with a tagline, the existing badges plus a Releases badge, and navigation links.
• A short pitch built around the four strategies (prove, check, guard, record), then a Highlights list.
• A new How it works section with a table of the six ledger outcomes and how weavec-cc enforces each; annotations get their own subsection.
• Using it as the compiler split into Checks, Guards and Analysis without building.
• Modes and options: fixes a stray blank line that split the bullet list; the run-on "other flags" sentence becomes a table, with a link to the CLI reference.
• Guarantees: (S)/(N)/(T)/(G)/(V) wording kept, now labelled; the six assumptions as a table; the limits of (G) as a list.
• Installing a release as numbered steps; a new Documentation index; the Code of Conduct linked from Contributing.

Design notes

• Every code example and every block of compiler output is byte-for-byte unchanged (one comment in the build block: # Everyone → # All platforms).
• Removed "The first release creates CHANGELOG.md…", which is stale now that CHANGELOG.md exists; the README links to it instead.
• The ## Building heading is kept because CONTRIBUTING.md links to README.md#building.

Test plan

• Diffed all fenced blocks against the previous README: identical apart from the comment above
• Every relative link resolves to an existing file
• Rendered page checked on GitHub

Documentation only; no code, tests or formats change.