[Documentation] Add new Getting started and Configuring the UI pages.

[miguelandres]

-Add a Getting Started page that explains dependencies and their nuances, initialization of Remixer, Variable definition and UI set up. -Add a Configuring the UI page that explains UI setup in more detail and with code examples. -Move the build system details to project structure page

This is more of the multi-part documentation CL started in #124:

• Project Structure (#124, it also adds a high level summary for each project and their key pieces)
• Getting Started and dependencies (This CL).
• Remixer concepts and supported variable types
• UI configuration (This CL, multiple ways to load the ui, these docs do not exist yet)
• Sync and storage configuration (This CL, in the Getting Started coc)
• Configuring the Firebase remote controller (still not done)
• Extending remixer (it's possible but not even discussed in current docs)

[codecov-io]

Codecov Report

Merging #125 into develop will not change coverage. The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #125   +/-   ##
==========================================
  Coverage      61.26%   61.26%           
  Complexity       233      233           
==========================================
  Files             53       53           
  Lines           1402     1402           
  Branches         136      136           
==========================================
  Hits             859      859           
  Misses           487      487           
  Partials          56       56

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