docs: curate the HexDocs module sidebar for 1.0

[davydog187]

Why

Going into 1.0, the HexDocs sidebar rendered ~90 module pages flat and alphabetical — the entire Lua.AST.* node tree, VM/compiler/parser/lexer internals, and five internal *Error types easily confused with the two users actually rescue. Root cause: docs: had no groups_for_modules and no module filtering, so every module without @moduledoc false leaked into the sidebar (only 5 of 60 were hidden).

What

Curated the published module surface to the ~8 modules a library user touches, via filter_modules + groups_for_modules:

Group	Modules
Core	Lua, Lua.API, Lua.Table, Lua.Chunk
Errors	Lua.RuntimeException, Lua.CompilerException, Lua.Parser.Error
Mix Tasks	mix lua.eval

Internals (Lua.AST.*, Lua.VM.*, Lua.Compiler.*, Lua.Lexer, Lua.Parser, …) are filtered from the published build only — their @moduledoc stays intact for source readers and h Mod in IEx. Nothing is deleted.

Restructured extras into Guides (Working with Lua · Security & Sandboxing · Mix Tasks) and Examples (the six Livebooks), and wired in the previously orphaned guides/mix_tasks.md.

Content fixes:

• README's Examples section pointed at examples/*.exs files that became Livebooks in 5d0d8fd → repointed at guides/examples/*.livemd; dropped the stale 1.0.0-rc.0 sentence; disambiguated the two sandboxing links.
• Lua.API: fixed the {: .neutal} → .neutral admonition, the deflua example that bound state but referenced lua, and three typos.
• working-with-lua.livemd: 0.4.0 → 1.0.0-rc pin, filled the empty ~LUA cell.
• sandboxing.md: coroutity and instruction_count typos.

Notable decisions

• Lua.Parser is treated as internal. Its public @specs return internal AST types (Lua.AST.Chunk, Lexer.token()), so documenting it drags the internals back into the sidebar. The structured-parse feature for tooling authors is represented by the kept, documented Lua.Parser.Error (its moduledoc names parse_structured/1 as the entry point). Also inlined Lua.Parser.Error.position/0 so the public wire type no longer leaks Lua.AST.Meta.
• ROADMAP.md is not shipped to HexDocs. It's an internal strategy doc (plan IDs, PR numbers, triage state) and produced broken-link warnings for .agents/plans/ paths. The README's "Coverage and status" already gives users an honest compatibility picture. A purpose-written guides/compatibility.md is an easy follow-up if we want a dedicated page.

Verification

• mix docs — zero warnings; sidebar confirmed at the 8 curated modules.
• mix test — 2523 passed (64 doctests), 7 skipped.
• mix compile --warnings-as-errors and mix format --check-formatted clean.

No source behavior changes — docs/config only.