fix: document the C compiler and CMake prerequisite for source builds

[MaximeGaudin]

Summary

• Building from source silently requires a C compiler and CMake: the TLS stack links aws-lc-sys, which has hard build-dependencies on cc and cmake and compiles AWS-LC's C sources at build time. Neither was documented.
• Verified the backend cannot be switched to ring from this repo: reqwest 0.13.4 (pulled by genai) hardcodes the aws-lc-rs provider in its rustls feature, while ureq (and extism) use the ring provider — both are additive, so rustls builds with both providers. There is no consumer-side feature to drop aws-lc-rs, so per the issue this is documented rather than a dependency change.
• Documented the prerequisite in both source-build entry points: the README "Development" section and the docs site's contributing page ("Getting it running"), with per-OS hints.

Fixes #192

Test plan

• pnpm exec turbo run lint --filter=slint-docs (biome + astro check + no-todos) and pnpm exec turbo run build --filter=slint-docs (48 pages) pass in the worktree.
• CI green on the PR