Add breadcrumbs to all sphinx pages

[Andy-Grigg]

Closes #16

Summary:

• Enable breadcrumbs with the html_theme_option "show_breadcrumbs": True
• The breadcrumbs appear on the page body itself as opposed to the navbar. I might have preferred to have them on the navbar so they are always visible, but I think that would require a major redesign of the theme
• The new css overrides some bootstrap defaults, so it was necessary to ensure it is added after the bootstrap css
• Additional 'root' breadcrumbs can be added, e.g. to the pyansys root page with the html_theme_option "additional_breadcrumbs": [("Link text", "href")]

Issues:

• The breadcrumb that links to the module homepage has to be added manually when on a page within the docs. This is fine, but the problem is there's no way to get the title of the index.rst page. As a result, I'm relying on using the html_title as set in conf.py. The problem is that this isn't necessarily the same as the title of the index.rst page.

Todo:

Assuming the approach is accepted, the docs need to be updated to describe how to enable this functionality and how to correctly set the html_title as described above.

Screenshots

Docs without any changes to conf.py:

With breadcrumbs added: "show_breadcrumbs": True,

With additional root element linking to pyansys: "additional_breadcrumbs": [("PyAnsys Home", "https://docs.pyansys.com/")],

[akaszynski]

This is excellent work and solves the navigation problem.