Complete doc overhaul

[lorenzwalthert]

This PR improves documentation in various ways, in particular:

• README:
  • moves adoption and examples out to vignettes.
  • move feature overview to intro vignette.
  • drop contributing guidelines.
  • updates badges (gh actions, travis appveyor etc).
  • add motivation
• big review of intro to styler vignette and rebranding as 'Get started' article with direct link in navbar, in particular:
  • caching
  • stylerignore
  • dry mode
  • scope specification
  • alignment detection
• caching and alignment: better cross-linking and documentation.
• add vignette with examples for strict = FALSE.
• add jupyter lab integration to adaption vignette.

[jonmcalder]

I've suggested a few small changes but overall it looks good to me!

[codecov-io]

Codecov Report

Merging #741 (79cd848) into master (52b70fd) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #741   +/-   ##
=======================================
  Coverage   90.50%   90.51%           
=======================================
  Files          47       47           
  Lines        2381     2383    +2     
=======================================
+ Hits         2155     2157    +2     
  Misses        226      226           

Impacted Files	Coverage Δ
R/nest.R	100.00% <ø> (ø)
R/style-guides.R	99.39% <ø> (ø)
R/ui-caching.R	100.00% <ø> (ø)
R/zzz.R	0.00% <0.00%> (ø)
R/stylerignore.R	100.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 52b70fd...94741c3. Read the comment docs.

[lorenzwalthert]

Thanks @jonmcalder 🥳