Release prep: bump to v0.2 + refresh docs (#807)

[DavidObando]

Release prep for the v0.2 line ("Oats" sweep, parent epic #706). This PR bumps the nbgv version base, refreshes the docs to match the v0.2 ground truth, and rotates the Docusaurus versioned-docs snapshot from 0.1 to 0.2. The orchestrator will tag v0.2.<patch> on main once this merges.

What's in this PR

Three commits, one logical change each:

1. chore — version.json bumped from 0.1 to 0.2 (patch component remains auto-derived from commit height by nbgv).
2. docs — release notes rewritten with a dated 0.2 section, diagnostics catalogue extended (GS0285–GS0297 added; GS0365 added; ADR-0102 covered alongside ADR-0101), a stale "v0.1 design" comment removed from a tutorial.
3. docs(website) — fresh version-0.2 snapshot generated via npm run docusaurus -- docs:version 0.2; old version-0.1/ snapshot, sidebar, and versions.json entry retired.

v0.2 themes covered in the release notes

Language additions
while / do…while + labeled break/continue (ADR-0070), if let / guard let (ADR-0071), ??= (ADR-0072), a?[i] (ADR-0073), arrow lambdas (ADR-0074), (T) -> R function-type clause (ADR-0075), lambda binding inference (ADR-0076), Kotlin/Swift-style type-declaration grammar (ADR-0078), if-as-expression finished (ADR-0064, #711), smart-cast extensions (ADR-0069 addendum), discriminated-union enum payloads (#725), default(T) (ADR-0100), variadic ...T params at every declaration site (ADR-0101 + ADR-0102 + ADR-0103), class / struct / new() constraints (ADR-0097), default-interface methods (ADR-0085/0089/0090/0091), reified generics (ADR-0087), Gsharp.Extensions.Optional / Sequences (ADR-0084), friendly numeric aliases (ADR-0098), map[K,V] (ADR-0104).

Removals
type keyword for type declarations (ADR-0078), record keyword (ADR-0078), := short declaration (ADR-0077, GS0305), name = value named-arg form (ADR-0080, GS0315), func(T) R legacy function-type syntax (ADR-0075, GS0303), Go-flavored map[K]V (ADR-0104, GS0366).

Native interop end-to-end
@DllImport (ADR-0086), @LibraryImport (ADR-0092), struct marshalling (ADR-0093), ref/out/in (ADR-0094), function pointers (ADR-0095), @MarshalAs (ADR-0096).

Concurrency
Go-flavored go / chan / select and built-ins len / cap / append / make / delete gated behind import Gsharp.Extensions.Go (ADR-0082, ADR-0083).

Tooling
LSP completion polish for async-shaped types (#713), nil-related quick fixes from textDocument/codeAction (ADR-0099), null → nil "did you mean" diagnostic (ADR-0081, GS0273).

Verification

• dotnet build GSharp.sln -nologo -clp:NoSummary — succeeded (warnings only; baseline MSB3026 retry + GS9100 reference closure warning on the Extensions project).
• dotnet test GSharp.sln --no-restore --nologo — all green: 32 + 104 + 296 + 266 + 3262 + 1319 = 5279 passed, 0 failed, 1 skipped (existing RegenerateBaseline).
• dotnet tool restore — dotnet-ilverify restored.
• cd website && npm ci && npm run build — clean build, no broken links, no warnings.
• grep '"version"' version.json → "version": "0.2",
• website/versioned_docs/version-0.1/ is gone, version-0.2/ exists; versions.json lists only "0.2".

Follow-up (orchestrator)

After merge, the orchestrator will:

1. nbgv get-version on the merged main to compute the patch component.
2. git tag -a v0.2.<patch> and push the tag; the publish job in .github/workflows/build.yml handles the release artefacts.
3. Close G# Language — Current State and Design Opportunities #706 (parent epic) once the tag ships — this PR does not close G# Language — Current State and Design Opportunities #706.

Closes #807.
Parent: #706 (will be closed after the v0.2 tag is pushed).