docs: add "Reducing closure size" tutorial (gcc closure leak)

[hamishmack]

What

Adds a new tutorial page, Reducing closure size (removing the gcc dependency), under docs/tutorials/, and wires it into the mdBook SUMMARY.md navigation.

The page documents the existing opt-in knobs for trimming the compiler toolchain (most visibly gcc, ~143 MB) out of an executable's runtime closure:

• nix why-depends for diagnosing what pulls gcc in;
• per-component dontStrip = false; + dontPatchELF = false; for dynamically linked executables (with a copy-pasteable modules override, scoped to the exes component);
• remove-references-to / nukeReferences in a postInstall for fully static binaries, where store paths remain baked in as string data;
• an honest note on why these are opt-in.

Defaults are intentionally unchanged

This PR is documentation only. The defaults (dontStrip = true, dontPatchELF = true in modules/component-options.nix, with builder/comp-builder.nix passing --disable-executable-stripping / --disable-library-stripping accordingly) are deliberately left as-is.

Stripping has historically been reported to occasionally break Haskell binaries (angerman in #336), and hamishmack's follow-up question in the thread — whether that still holds with current GHC/binutils — was never answered. Flipping the default without resolving that would be unsafe, so this documents the safe opt-in path and tells users to test their binaries after enabling it.

Flipping the executable-only default (nixpkgs strips executables by default; the historical breakage was about aggressive stripping of library object files used for further linking) could be a sensible follow-up once maintainers resolve the stripping-safety question.

Verification

• Verified option names/behaviour against the actual source (modules/component-options.nix, builder/comp-builder.nix).
• Built the docs with mdBook: the new page renders and appears in the nav. (The pre-existing book.toml multilingual field is rejected by current mdBook and the only build warnings come from the pre-existing dev/coverage.md; both are unrelated to this change.)

Closes #829.