Improve navigability

[degville]

Update current documentation structure to improve the following:

• general navigability; current split of top level headings (ie. Introduction/Nodes/API) isn't optimal.
• new user on-boarding and discovery; There's currently no obvious path through the docs, especially when a reader may be unfamiliar with the concepts and terms. Add Get Started section.
• added walkthroughs; maybe within their own section. There should a quickstart walkthrough to cover the general installation and use, plus more hand-on guides for common tasks.

Steps to solve this issue:

• Create updated table of contents to improve navigability #749 Create updated table of contents to improve navigability
• Ensure there's a clear path through the early parts of the docs #750 Ensure there's a clear path through the early parts of the docs
• Add 'Expore MAAS' document #751 Add 'Explore MAAS' doc
• Update documentation to reflect new table of contents #752 Update documentation to reflect new table of contents
• Populate a new MAAS quickstart section #753 Populate a new MAAS quickstart section

#742 is a related issue.