Improve Landing Page and Quickstart Guide

[callumforrester]

Addresses some of the concerns raised in #279

Changes:

• Add concept explanation and diagram to documentation landing page
• Add brief description and testing instructions to quickstart guide

[codecov]

Codecov Report

Merging #292 (d6229cc) into main (1dd6b0f) will not change coverage.
The diff coverage is n/a.

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##             main     #292   +/-   ##
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  Coverage   89.44%   89.44%           
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  Files          41       41           
  Lines        1412     1412           
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  Hits         1263     1263           
  Misses        149      149           

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[callumforrester]

Preview of landing page here: https://github.com/DiamondLightSource/blueapi/tree/welcome-page
Preview of docs changes here: https://diamondlightsource.github.io/blueapi/welcome-page/index.html

[DominicOram]

Great, work, thanks. Not strictly part of this PR I guess but the CLI commands on Quickstart->Start Worker do not work for my latest install of blueapi

[callumforrester]

@DominicOram The CLI commands work for me, could you confirm which version of blueapi you are using?
These commands should work for >=0.3.0

[DominicOram]

My bad, all good with latest blueapi version