Feat: fix sidebar to the page for easy navigation

Hello there! I was reading through the InfluxData Docs and something caught my eyes. I noticed that the current the sidebar isn't fixed to the page. From my experience and from most documentation sites I've found on the web, a best practice is to have the sidebar fixed to page i.e it shouldn't scroll along with the reader as they scroll across the page.

With the current sidebar, when a reader scrolls to the bottom of a very long page and wants to navigate to another page linked on the sidebar. It will have them scrolling all the way to the top, trying to find the page on the sidebar. That might not be ideal in every situation.

I propose that the sidebar gets fixed to the page to address the issues above.

##### Relevant URLs
- _Provide relevant URLs_

Start by locating the documentation site's sidebar implementation and inspect how it behaves while a reader scrolls through a long page. The work is done when the sidebar remains available for navigation without requiring the reader to return to the top; verify the behavior on a sufficiently long documentation page.

Written by the indexing model from the issue text.