docs: Verify HexDocs generation and public API documentation

[zacky1972]

Summary

Improve HexDocs readiness for the v0.1.0 public preview API.

This PR keeps the existing v0.1.0 release scope and package metadata intact, then strengthens the generated documentation for first-time Hex users:

• expands LeanLsp docs around the narrow v0.1.0 runtime-preview API;
• documents LeanLsp.runtime_config/1 as side-effect-free option normalization;
• documents LeanLsp.start_runtime/1 as Docker-backed runtime startup with caller cleanup responsibility;
• expands LeanLsp.Runtime behaviour docs, types, callback return shapes, and runtime side-effect expectations;
• adds public docs for all LeanLsp.Runtime.Config functions;
• makes LeanLsp.Runtime.Docker.start_link/1, stop/1, and exec/3 visible in ExDoc despite being behaviour implementations;
• documents Docker startup, docker exec command execution, container cleanup, external Docker requirements, runtime options, mounts, env vars, and timeouts;
• adds ExDoc module and extras grouping to improve generated HexDocs navigation.

Notes

mix docs already completed without warnings before this change, but the generated docs showed two gaps:

1. LeanLsp.Runtime.Config public functions rendered mostly as specs without explanatory docs.
2. LeanLsp.Runtime.Docker callback implementations were hidden from the module page because @impl marks callback implementations as undocumented unless @doc is explicitly provided.

This PR addresses both gaps without changing README content or the README ## License section.

Validation

Run from a branch that contains the latest origin/main:

mix format --check-formatted
mix docs --warnings-as-errors
mix check

Manual docs review checklist:

• LeanLsp explains that v0.1.0 is a runtime-preview release, not a complete Lean LSP client.
• LeanLsp.Runtime.Config has docs for default_runtime/0, default_docker_image/0, default_container_workspace_root/0, normalize/1, and to_runtime_options/1.
• LeanLsp.Runtime.Docker shows child_spec/1, start_link/1, stop/1, and exec/3 with side-effect and cleanup notes.
• README and extra docs still appear in generated HexDocs navigation.