Add Next and Back Navigation Buttons to the Documentation

[corwinjoy]

Add navigation buttons to the documentation as per the proposal here: #2852

Comments:

1. I was able to get this to work pretty well but there were a couple limitations.
   a. The Sitemap appears twice, once at the top level and once at the beginning of the documentation. I had to pick one to get a single navigation order so I chose the Sitemap position in the left menu.
   b. I did not include navigation for the demo page since I think that makes no sense.

2. I made the buttons as pretty as I know how, but I'm not all that great at graphic design. The buttons are a class in the css file so I welcome suggestions there.

3. It might make sense to have navigation buttons at the top of the page as well, but I am not as sure how, or if, I should do that.

Thanks in advance for taking a look! I do think this does help to improve the docs.

[corwinjoy]

I did also run the linter as suggested by the contribution guide. I didn't see any errors from this addition, but it did come back pretty dirty / suggesting changes for the existing code so I'm not sure what to make of that. (Edit: Regex errors already seem to be fixed and I have fixed the reported grammar errors in a separate PR.)

[corwinjoy]

As mentioned, this page is pointed to by both the the docs and the sitemap link. I had to choose a single location in order to get a unique navigation order.

[szarnyasg]

Hi, thanks for this PR! I passed it along to our design team for feedback. Will let you know the outcome.

[corwinjoy]

@szarnyasg Very cool! Does the design team have any questions or feedback? I think this would be a helpful feature for navigating the web documentation.