Docs: Spack Env (#1348)

Adding a simple spack environment to install all dependencies to
build the documentation locally. This is just an alternative to the
already provided (pip) `requirements.txt` file (prequisite: have
doxygen installed).

Activating and installing this environment will provide *all*
dependencies (incl. Doxygen) that are needed to build the whole
documentation locally. Spack manual link:
  https//spack.readthedocs.io/en/latest/environments.html#anonymous-environments

Inside WarpX' `Docs/` directory:
```
   spack env activate -d .
   spack install  # only needed the first time
```

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