chore(virtual-scroll): use pf4 css grid layout for docs

[priley86]

What:
I noted in #1707 in OpenShift that our PF4 Grid Layout can be used to replace a BS3 Flexbox grid (or an emulation of that) in this scenario. We are deferring this in OpenShift for now until it can be done globally, but I am just updating the documentation here w/ best recommendation for anyone else.

• note: also updates our Grid docs CSS selector here to be more specific so that pf-l-grid can be used without the dotted dash lines in other component examples)

Additional issues:

[patternfly-build]

PatternFly-React preview: https://2255-pr-patternfly-react-patternfly.surge.sh

[codecov-io]

Codecov Report

Merging #2255 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2255   +/-   ##
=======================================
  Coverage   79.89%   79.89%           
=======================================
  Files         669      669           
  Lines        8529     8529           
  Branches      734      734           
=======================================
  Hits         6814     6814           
  Misses       1362     1362           
  Partials      353      353

Flag	Coverage Δ
#patternfly3	85.23% <ø> (ø)	⬆️
#patternfly4	74.92% <ø> (ø)	⬆️
#patternflymisc	95.79% <ø> (ø)	⬆️

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[tlabaj]

LGTM