🎨 Update docs theme and add tutorial

[chrisjsewell]

• Update the theme to furo and colors to match new branding
  • branding coloring loosely based on https://mintlify.com/docs/quickstart
• Restructure toc-trees, to make the documentation easier to navigate
• Improve landing page
  • Improve top "hero" section
  • Add feature section
  • move examples to tutorial
• Add tutorial page, with worked example covering most directives
  • took some things from https://sphinx-needs-demo.readthedocs.io

https://sphinx-needs--1184.org.readthedocs.build/en/1184/

---

Note, one last thing that I would like to do,
is to add a clear indication that this is "powered by useblocks",
with a link to the main useblocks site (and enteprise tools) on the landing page and "next steps" of tutorial.
But this is waiting on the new site being released.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.44%. Comparing base (1de1a7a) to head (b2389e0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1184   +/-   ##
=======================================
  Coverage   86.44%   86.44%           
=======================================
  Files          56       56           
  Lines        6501     6501           
=======================================
  Hits         5620     5620           
  Misses        881      881           

Flag	Coverage Δ
pytests	86.44% <ø> (ø)

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