docs(Gauge): add documentation to Gauge

[Valentin271]

Adds documentation to Gauge and LineGauge widgets.

Relates to #386

Screenshots

[codecov]

Codecov Report

Merging #514 (915e1e4) into main (0c7d547) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #514   +/-   ##
=======================================
  Coverage   89.93%   89.93%           
=======================================
  Files          41       41           
  Lines       11334    11334           
=======================================
  Hits        10193    10193           
  Misses       1141     1141           

Files Changed	Coverage Δ
src/widgets/gauge.rs	92.42% <ø> (ø)

... and 2 files with indirect coverage changes

[joshka]

Docs are good. Could have a bit more info, but I'd merge this unless you want to make the suggested changes.

[joshka]

Thanks for making the changes. This looks great!