Restructure documentation to make sidebar look more informative

[jaladh-singhal]

Problem description

Current structure in docs doesn't organizes pages in a way that is easy to find. We need to make changes in hierarchical changes in TOC, so that sidebar in https://tardis-sn.github.io/tardis/ becomes informative enough to get a gist of important pieces of content at one glance!

Expected Solution

Required hierarchy in sidebar:

• 1st level entries with "main title" refers to the top level heading that appears in blue - like "Using TARDIS" at sidebar on https://tardis-sn.github.io/tardis/
• 2nd level bullets with "shown in scrollbar" refers to the items that directly appear under them (in white) - like "Running TARDIS" at sidebar on https://tardis-sn.github.io/tardis/
• 3rd level bullets refer to the contents that appear only when clicking on a 2nd level item (above) redirects us to its page and then sidebar expands to show them - like try clicking "Running TARDIS" you'll reach https://tardis-sn.github.io/tardis/running/index.html

Most of these changes will only need to change TOCs but some may require to actually move rst files from one directory to another under docs/