Support Storybook 7 and docs pages

[prantlf]

• Adapt the interface to be able to set up the custom sorting in Storybook 7 too, by a combination of changes in main.js and preview.js.
• Offer sorting the pages to groups by their type - docs and story.
• Add an article about how to migrate the configuration from the version 1 to 2.
• Migrate the project example.

BREAKING CHANGE: Only Storybook 7 is supported. If you use Storybook 6 or older, stay with the versions 1.x. Also, the minimum supported version of Node.js is 16. Except for the version requirements, the configuration of the custom sorting needs to be specified by a different interface. See the documentation about migration for more information.

[codecov[bot]]

Codecov Report

Merging #10 (467c924) into master (ba889a2) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #10   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           78       146   +68     
=========================================
+ Hits            78       146   +68     

Impacted Files	Coverage Δ
lib/index.js	100.00% <100.00%> (ø)

[prantlf]

I decided to go on with this variant. Who knows when I figure out the simpler solution with the addon options.