make docs: don't crash on non-existing autogenerated files

The autogenerated index and module index might not exist in a new project. 

Attempting to remove them will cause an error if they don't exist, so we pass the -f flag to rm to ignore the error.
make docs: Use project root as first element in PYTHONPATH

When we use `sys.path.append(parent)`, `parent` is the last element in the path. 

Consequently, if the module we're building docs for is already installed, then that one will be installed, and `version = module.__version__` will use the version of the installed module instead of that of the source package.

Inserting the project root as first element in the PYTHONPATH lets us ensure that the source package is imported, and that its version is used.
Merge pull request #4 from krallin/patch-1

make docs: don't crash on non-existing autogenerated files
Fix some minor CONTRIBUTING formatting errors
Add lucuma/cookiecutter-pypackage
Clarification to answer questions: now Fork This / Create Your Own
Link to fork network and family tree.
Merge branch 'correct-version' of git://github.com/krallin/cookiecutter-pypackage into krallin-correct-version
Rename parent var to project_root, and add comments as per @krallin's explanation.
Update CONTRIBUTING.rst

Small syntax corrections
Merge pull request #9 from lgiordani/master

Update CONTRIBUTING.rst
Added basic wheels support
Forgot sdist, whoops!
typos
Update the default dates to 2014.
Add htmlcov to .gitignore
Remove htmlcov when cleaning up
Remove broken link to lucuma template package.
Add my template fork.
Merge pull request #13 from solarnz/updated-date

Update the default dates to 2014.