docs: add doc.go and rewrite exported godoc (#11)

[millerjp]

Summary

Moves package-level documentation into a dedicated doc.go and rewrites the godoc on every exported symbol in syncmap.go. The inline placeholder package comment added in #8 (solely to satisfy revive.package-comments) is removed.

Closes #11.

doc.go covers

• Overview — what syncmap is, why it exists, assertion-free call sites.
• Relationship to sync.Map — thin typed wrapper, API parity, same concurrency guarantees.
• When to use SyncMap vs sync.Map vs map + sync.RWMutex — with guidance bullets.
• Thread safety — inherited from sync.Map.
• Zero value — usable, must not be copied after first use.
• Quick Start — compiles mentally, uses tab-indented code block per godoc contract.

Per-symbol godoc rewrites (syncmap.go)

Every exported symbol now starts with its name and matches or improves on sync.Map's stdlib prose:

• Len, Map, Keys, Items — each explicitly documents O(n) cost and "point-in-time approximation" under concurrent mutation.
• LoadAndDelete — explicitly documents the typed-zero-value-on-miss contract (was returning nil/untyped pre-Go-generics wording).
• Range — preserves stdlib sync.Map Range snapshot wording so readers familiar with stdlib recognise it.

Local validation

$ make check
… (all 14 gates pass)
coverage: 100.0% of statements
No vulnerabilities found.

$ go doc github.com/axonops/syncmap
Package syncmap provides a type-safe, generic wrapper around sync.Map.
… [full overview rendered]

$ for s in Len Map Keys Items; do go doc github.com/axonops/syncmap $s | grep 'O(n)'; done
    It runs in O(n) time by traversing the map with Range.
    in O(n) time.
    call. It runs in O(n) time.
    call. It runs in O(n) time.

$ go doc github.com/axonops/syncmap LoadAndDelete | grep 'zero value'
    was not present, value is the zero value of V.

Agent gates

• Plan — execution plan for single-commit docs-only PR.
• docs-writer (pre-code) — produced the draft content.
• code-reviewer — approved; zero behaviour change confirmed (diff is pure comment + new file).
• docs-writer (post-code) — PASS on all eight doc-standard criteria.
• go-quality — 14/14 green.
• commit-message-reviewer — PASS.

Out of scope

• README, examples, BDD scenarios — owned by later issues (docs: add README, CHANGELOG, NOTICE, SECURITY, and LLM docs bundle #17, test: add benchmarks, runnable examples, and bench-regression infrastructure #15, test: add BDD (godog) suite and fuzz targets #16).
• Any code / API / signature / behaviour change.

Test plan

• CI green end-to-end (all guards, lint, tests, build matrix, security, release-check)
• Coverage remains 100%
• No regression on any quality gate