split README into topic-based guides - #16

This pull request reorganizes the Microsoft C++ Language Server documentation into topic-based guides and reference pages under docs/. The root README stays focused on the project overview, full inline quick start, essential links, and legal notices.

Documentation changes

• Moves detailed installation, authentication, compilation-database, configuration, indexing, troubleshooting, command-line, and LSP-feature documentation into dedicated pages.
• Adds docs/index.md as the documentation index, organized into Guides and Reference. The root README remains the only file named README in the repository.
• Preserves existing README section links through compatibility anchors beside the relevant replacement topic links, and updates cross-links throughout the documentation.
• Adds guidance for disabling and re-enabling WCI through the per-user state file, including its scope, restart requirements, trade-offs, and precautions for preserving existing settings and authentication data.
• Documents --check=<file> and --check-compile-commands=<path>, including standalone usage, prerequisites, compilation-database discovery, and check-mode exit codes.
• Keeps extractor-authoring guidance linked from the compilation-database guide, removes unrelated resource links and maintenance instructions from the index, and retains feedback and data-collection guidance.

Manual validation

• All 16 repository Markdown files render, and all 62 local links resolve.
• All three JSON examples parse; legacy README anchors and the quick-start/WCI ordered-list structure are preserved.
• Documentation whitespace checks pass.

This PR changes documentation only; it does not change language-server or plugin runtime behavior.