Start a "How to contribute" guide in the docs

Currently the only developer resources are [C++ guidelines](https://docs.fenicsproject.org/dolfinx/v0.8.0/python/developer.html).

I recently wanted to test a PR and used this dockerfile to create a container within which I could run some tests.

```dockerfile
FROM ghcr.io/fenics/dolfinx/dolfinx:nightly

ARG CMAKE_NUM_PROCESSES=4
ARG DOLFINX_MODE=real
ARG DOLFINX_PRECISION=64
ARG DOLFINX_CMAKE_CXX_FLAGS="-march=native"
ARG BUILD_TYPE=Developer

ENV DOLFINX_DIR=/usr/local/dolfinx-${DOLFINX_MODE}${DOLFINX_PRECISION}
WORKDIR /src

RUN git clone --branch=dokken/improve-mixed-packing --single-branch https://github.com/fenics/dolfinx.git
RUN CMAKE_BUILD_PARALLEL_LEVEL=${CMAKE_NUM_PROCESSES} cmake -G Ninja -B build-dir-${DOLFINX_MODE}${DOLFINX_PRECISION} -DCMAKE_INSTALL_PREFIX=${DOLFINX_DIR} -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DCMAKE_CXX_FLAGS="${DOLFINX_CMAKE_CXX_FLAGS}" -S ./dolfinx/cpp

RUN CMAKE_BUILD_PARALLEL_LEVEL=${CMAKE_NUM_PROCESSES} cmake --build build-dir-${DOLFINX_MODE}${DOLFINX_PRECISION}
RUN cmake --install build-dir-${DOLFINX_MODE}${DOLFINX_PRECISION}
RUN python3 -m pip -v install --check-build-dependencies --config-settings=build-dir="build-${DOLFINX_CMAKE_BUILD_TYPE}-${DOLFINX_MODE}-${DOLFINX_PRECISION}" --config-settings=install.strip=false --config-settings=cmake.build-type="${DOLFINX_CMAKE_BUILD_TYPE}" --config-settings=cmake.define.CMAKE_BUILD_PARALLEL_LEVEL=${CMAKE_NUM_PROCESSES} --no-build-isolation -e ./dolfinx/python/
```

We could adapt this Dockerfile to install any branch/commit to easily test PRs.

Would it be worth adding a section to Developer resources with this as an alternative to build everything locally?
What do people think?