Make GMP an optional dependency; add developer documentation

[loumalouomega]

Summary

GMP (GNU Multiple Precision) was previously a hard build requirement that caused cmake to abort when the library was not installed. This PR makes GMP opt-in and ships a full documentation set and a convenience configure script.

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Changes

GMP is now optional (FLOAT_TETWILD_USE_GMP)

• CMakeLists.txt — added a new CMake option FLOAT_TETWILD_USE_GMP (default OFF). When disabled, find_package(GMPfTetWild) is skipped entirely and GMP headers/libraries are never linked. When enabled, the existing GMP path is used and the preprocessor macro FLOAT_TETWILD_USE_GMP is propagated to the library.
• Rational.h — the Rational class is now conditionally compiled:
  • With -DFLOAT_TETWILD_USE_GMP=ON: full GMP-backed mpq_t exact rational arithmetic (previous behaviour).
  • Without (default): a lightweight long double fallback implementation is used for AMIPS energy stabilisation, removing the hard GMP dependency for the common case.

New configure script (configure.sh)

A self-contained Bash helper that wraps cmake with a structured argument parser. Features include:

• All fTetWild feature flags exposed as named options (--with-gmp, --no-tbb, --exact-envelope, --use-float, …).
• Compiler, sanitizer (ASan / MSan / TSan / UBSan), and dependency-path overrides.
• Auto-detects Ninja vs. Unix Makefiles; prints a human-readable configuration summary before running cmake.
• --dry-run mode prints the assembled cmake command without executing it.
• --offline sets FETCHCONTENT_UPDATES_DISCONNECTED=ON for air-gapped builds.

Documentation (doc)

Nine new Markdown files documenting the algorithm and codebase:

File	Content
index.md	Entry point and navigation
overview.md	High-level algorithm overview and pipeline
preprocessing.md	Input surface preparation
triangle_insertion.md	Triangle insertion into the tetrahedral mesh
mesh_improvement.md	Local mesh improvement operations
data_structures.md	Core data-structure reference
filtering_and_booleans.md	Inside/outside filtering and CSG booleans
envelope_and_aabb.md	Envelope and AABB wrapper details
dependencies.md	Third-party dependencies overview

README.md was updated to reference the new docs, clarify that GMP is optional, and describe the --with-gmp configure flag.

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Migration / upgrade notes

• Existing users who rely on GMP exact arithmetic: pass -DFLOAT_TETWILD_USE_GMP=ON to cmake (or --with-gmp to configure.sh) — behaviour is identical to before.
• New users / CI without libgmp-dev: no change needed; the build works out of the box with the long double fallback.