more introduction to Workplanes in the docs

[marcus7070]

I was trying to help a user (here) and I wanted to point them to somewhere in our docs where they could get the concept of a workplane and how solids are created relative to a workplane. I think they hadn't encountered the concept of workplanes before. I realised the docs don't say too much about the concept of workplanes (probably because back when they were written the link to FreeCAD was much stronger, so it was assumed knowledge). So I've added some more words to the docs. Do you think they help? I try to be brief because I know docs with too many words won't be read at all.

[codecov]

Codecov Report

Merging #386 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #386   +/-   ##
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  Coverage   93.31%   93.31%           
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  Files          19       19           
  Lines        4847     4847           
  Branches      483      483           
=======================================
  Hits         4523     4523           
  Misses        210      210           
  Partials      114      114           

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[adam-urbanczyk]

Thanks @marcus7070 , it does look useful.

[adam-urbanczyk]

Small docs modification, so just merging.

[jmwright]

Yes, thanks @marcus7070