Clean up README, contribution guide, and repository metadata

- REUSE.toml: fix the Hedley path and SPDX id (CC0-1.0), mark the docset
  icons as the public-domain JSON logo
- CITATION.cff: v3.12.0 was released on 2025-04-11
- FILES.md: list all workflows, describe the Meson option defaults
- README: ctest -LE, table of contents, typos, stale Android/MinGW advice,
  moved links, merge duplicate Thanks entries, list the analysis tools
  used in CI
- CONTRIBUTING: iterative parser, json_literals.hpp is generated, links
- .gitignore: backups and release outputs; .gitattributes: mark
  generated files
- labeler: label other build systems and .github documentation
- MODULE.bazel: add the module version

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/README.md

@@ -7,12 +7,12 @@ This URL provides the most recent documentation and also applies to previous ver
 functions is not removed; instead, it is marked as deprecated.
 
 If you want to view the documentation for a specific tag or commit hash, you can generate it locally as follows (example
-using tag `v3.10.2`):
+using tag `v3.12.0`):
 
 ```shell
 git clone https://github.com/nlohmann/json.git
 cd json
-git checkout v3.10.2
+git checkout v3.12.0
 make install_venv serve -C docs/mkdocs
 ```