Fix the version switcher for the docs

[StephenNneji]

version switcher should now show up next to the logo

[MikeSullivan7]

I've just tested this and I it shows like this for me:

Not quite sure why yet

[MikeSullivan7]

Im getting this in doc/source/conf.py:
os.environ.get('DOC_URL', '')=''
leading to:

The version switcher "switcher.json" file cannot be read due to the following error:
FileNotFoundError(2, 'No such file or directory')

The DOC_URL must not be set in my environment properly

[StephenNneji]

It needs a switcher. json which is generated on deploy in deploy.py. You can test by adding `os.environ['DOC_URL'] = 'https://rascalsoftware.github.io/RasCAL-2/' into the conf.py before line 22 so it grabs the switcher file from the ghpages

[MikeSullivan7]

Works as intended once `os.environ['DOC_URL'] = 'https://rascalsoftware.github.io/RasCAL-2/' was set in my local files.

[StephenNneji]

Thanks for the speedy review, much appreciated