Enhance documentation

[crazy-max]

Fixes #215

And also:

• Split documentation to a docs directory
• Add note about issue with local cache backend (keeps growing)
• Update GitHub cache workflow with temp fix
• Note about future GitHub cache backend
• Workflow example to use secrets

cc @tonistiigi @MarcelCoding @ahmadnassri

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

[ahmadnassri]

looks good, and a good description for the topic in #215

[tonistiigi]

After the first example can we had a section explaining setup-buildx and setup-qemu are optional and when they are needed. And maybe a link to a longer writeup in the advanced section explaining usage of driver and the main differences(multi-platform, load, cache-export).

[crazy-max]

@tonistiigi Thanks for your feedback. I will make some changes based on this.

[codecov-io]

Codecov Report

Merging #299 (a63b18d) into master (af867d4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #299   +/-   ##
=======================================
  Coverage   74.39%   74.39%           
=======================================
  Files           4        4           
  Lines         164      164           
  Branches       28       28           
=======================================
  Hits          122      122           
  Misses         22       22           
  Partials       20       20           

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