docs: add issue templates, contributing file

Signed-off-by: Slendi <slendi@socopon.com>

CONTRIBUTING.md

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+# Contributing to smath
+
+Welcome to `smath` contributors!
+
+This document contains a set of guidelines to contribute to the project. If you feel like there's a mistake or something can be improved to this document, feel free to propose changes in a pull request.
+
+## Philosophy
+
+- `smath` itself should be kept header only, in a single file.
+- `smath` should be simple and easy to use.
+
+## Development setup
+
+You can work with either Nix (recommended, same tooling used in CI) or a local CMake toolchain.
+
+### Option 1: Nix
+
+Make sure you have flakes enabled and then execute the following to enter the shell:
+
+```bash
+nix develop
+```
+
+You can also run `direnv allow` if you have [direnv](https://direnv.net/) installed and configured.
+
+### Option 2: Local toolchain
+
+Requirements:
+
+- C++23-capable compiler
+- CMake (3.15+)
+- Ninja (recommended)
+
+### Configure and build
+
+```bash
+cmake -S . -B build -G Ninja -DSMATH_BUILD_TESTS=ON -DSMATH_BUILD_EXAMPLES=ON
+cmake --build build
+```
+
+## Running tests
+
+```bash
+ctest --test-dir build --output-on-failure
+```
+
+## Code style
+
+This project uses `clang-format` to maintain a consistent code style. Before opening a pull request, format changed C++ files:
+
+```bash
+git ls-files '*.hpp' '*.cpp' | xargs clang-format -i
+```
+
+## Pull requests
+
+- Keep changes focused and scoped to one topic.
+- Add or update tests when behavior changes.
+- Ensure the project builds and tests pass locally.
+
+### Commit style
+
+Use the commit message format: `<category>: <brief>`.
+
+Allowed categories: `feat`, `fix`, `test`, `docs`, `ci`.
+
+Examples:
+
+- `feat: add as_matrix() to quaternion`
+- `fix: correct mat4 approx_equal ignoring a column`
+- `test: add tests for vector swizzle edge cases`
+- `docs: fix typo in README.md`
+- `ci: update nix build command in pull request workflow`
+
+If a commit fixes a tracked issue, include an issue-closing footer: `Closes: #<num>` (example: `Closes: #42`).
+
+## Reporting issues
+
+When opening an issue, include:
+
+- what you expected to happen
+- what happened instead
+- a minimal reproducible example
+- compiler and platform details