docs: full Doxygen API documentation for CIF handling

[CV-GPhL]

Summary

This PR adds complete Doxygen API documentation for Gemmi's CIF (Crystallographic Information File) handling layer, covering 8 headers:

• cifdoc.hpp — core CIF document model (Document, Block, Loop, Item, Column)
• cif.hpp — CIF parsing entry points (read_file, read_string, read_memory)
• read_cif.hpp — CIF reading with path/stream/memory overloads
• to_cif.hpp — CIF writing (WriteOptions, write_cif_block_to_stream, write_cif_to_file)
• to_json.hpp — CIF-to-JSON conversion (JsonWriteOptions, write_as_json, write_as_mmjson)
• json.hpp — CIF document reading from JSON (cif_document_from_json)
• numb.hpp — CIF number parsing utilities (as_number, as_int, is_numb)
• ddl.hpp — DDL1/DDL2 dictionary validation (Validator, validate)

Documentation standard

• /// triple-slash Doxygen comments throughout
• Every public entity has @brief; non-obvious parameters have @param/@return/@throws
• Semantics verified against implementations (e.g., Loop::values row-major storage, numb ignores s.u., DDL1 vs DDL2 distinction)

Dependencies

This PR depends on PR #413 (Doxygen/Breathe infrastructure) being merged first.

It follows PR #414 (core data structures) in the documentation series.

Series context

This is PR 3 of a planned series of 10 PRs adding incremental API documentation:

PR	Branch	Content
#413	api-docs/infra	Build infrastructure (Doxyfile, conf.py, api.rst, readthedocs)
#414	api-docs/core-structures	model.hpp, unitcell.hpp, symmetry.hpp, metadata.hpp, elem.hpp, seqid.hpp, resinfo.hpp, small.hpp
This PR	api-docs/cif	cif.hpp, cifdoc.hpp, read_cif.hpp, to_cif.hpp, to_json.hpp, json.hpp, numb.hpp, ddl.hpp
PR 4	api-docs/structure-io	mmcif.hpp, mmread.hpp, pdb.hpp, to_mmcif.hpp, to_pdb.hpp, and related
PR 5	api-docs/reflection	mtz.hpp, refln.hpp, cif2mtz.hpp, mtz2cif.hpp, and related
PR 6	api-docs/maps-grids	grid.hpp, ccp4.hpp, dencalc.hpp, fourier.hpp, and related
PR 7	api-docs/calculations	calculate.hpp, neighbor.hpp, sfcalc.hpp, align.hpp, and related
PR 8	api-docs/chemistry	chemcomp.hpp, monlib.hpp, topo.hpp, and related
PR 9	api-docs/scattering-math	formfact.hpp, math.hpp, cellred.hpp, and related
PR 10	api-docs/io-utils	fileutil.hpp, gz.hpp, util.hpp, logger.hpp, and related

Attribution

Builds on and credits prior work in PR #402 (Paul Emsley / pemsley). All comments audited/rewritten where needed against actual implementations.