docs: fix bugs, wire orphaned pages into nav, refresh compiler and STL docs

[steven-varga]

Context

Paige audited docs/ and found a thin nav (4 published pages out of ~12 docs files), several orphaned architecture and example sub-pages, and a handful of content bugs. John, Sally, and Winston weighed in on scope and information architecture.

Immediate Bug Fixes

• docs/index.md — duplicate word: "With With LLVM-based" (line 6)
• docs/compiler.md — stale LLVM version reference (7.0.0); currently supported versions are Clang/LLVM 14, 15, 17 (per h5cpp-compiler CI matrix)
• docs/compiler.md — broken Doxygen-style @ref link_h5cpp_install link; replace with valid MkDocs-relative link

Nav Wiring (publish what already exists)

Wire the following orphaned files into mkdocs.yml:

• architecture-types.md → under Architecture
• architecture-integration.md → under Architecture
• architecture-parallelism.md → under Architecture
• compiler.md → top-level nav entry
• examples/attributes.md, examples/datasets.md, examples/groups.md, examples/container.md, examples/sparse-matrix.md → under Examples

architecture.md (top-level monolith) intentionally excluded until the type-engine refactor (branches 89, 92, 114) settles — per Winston.

Content Work

• docs/stl.md — rewrite from scratch; currently rough notes with typos and placeholder text ("code: 6487")
• docs/compiler.md — refresh prose to match current LLVM support and link to vargalabs/h5cpp-compiler

Information Architecture (Sally + John)

Target nav structure reflecting three user personas (evaluator, integrator, deep diver):

Overview | Design | Architecture | Type System | I/O Operations | Compiler Tool | Examples

Sally's recommendation: surface the examples/ sub-pages as real nav children, not a single flat page.

Out of Scope

• New content for architecture.md (blocked on type-engine refactor)
• Any changes to the MkDocs theme, CSS, or JS

Refs: #128 (README badge PR, same branch candidate)