fix plot size in png - rework docs

[t-bltg]

• fix incorrect plot size when exporting to png (buggy polarplot);
• cache font for speeding up png_image & savefig;
• add banner for pending Term 1.0.

[codecov-commenter]

Codecov Report

Merging #269 (665c3f2) into master (ecef83a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #269   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           29        29           
  Lines         1684      1686    +2     
=========================================
+ Hits          1684      1686    +2     

Impacted Files	Coverage Δ
src/canvas/dotcanvas.jl	100.00% <ø> (ø)
src/interface/polarplot.jl	100.00% <ø> (ø)
src/canvas/braillecanvas.jl	100.00% <100.00%> (ø)
src/plot.jl	100.00% <100.00%> (ø)

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[t-bltg]

We need to cache the fonts in order to bypass JuliaGraphics/FreeTypeAbstraction.jl#67.