Changes to subnets Getting Started and Overview guides

[mefrem]

Description

Updates to subnets documentation, specifically the Getting Started guide and Overview page, the result of feedback.

[codecov[bot]]

Codecov Report

Merging #261 (685914b) into master (7d373f0) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #261   +/-   ##
=======================================
  Coverage   93.47%   93.47%           
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  Files           6        6           
  Lines         337      337           
=======================================
  Hits          315      315           
  Misses         22       22           

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[mefrem]

Thanks Jeff, incorporated all your suggestions (and capitalized all instances of Stacks.js... I'm not sure why, but I think subnets has always been uncapitalized. Not sure why or who to confirm that with.)

I believe we're good to merge.

[mefrem]

Also here's a general Markdown tip: Write one sentence per line. It will be rendered the same, but it makes git diffs easier to read

cc: @LakshmiLavanyaKasturi — thanks Jeff!