Display maximum memory used during the site build

[caendesilva]

This was originally contributed by @rgasch in #667 and is cherry-picked to master to resolve #685 (comment)

[codecov]

Codecov Report

Merging #783 (7557e57) into master (dfe44a9) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##              master      #783     +/-   ##
=============================================
  Coverage     100.00%   100.00%             
- Complexity      1167      2334   +1167     
=============================================
  Files            142       284    +142     
  Lines           3100      6202   +3102     
=============================================
+ Hits            3100      6202   +3102     

Impacted Files	Coverage Δ
...ramework/src/Console/Commands/BuildSiteCommand.php	100.00% <100.00%> (ø)
...work/src/Framework/Features/Navigation/NavItem.php	100.00% <0.00%> (ø)
...work/src/Framework/Concerns/ValidatesExistence.php	100.00% <0.00%> (ø)
...mework/src/Framework/Services/DiscoveryService.php	100.00% <0.00%> (ø)
...mework/Features/XmlGenerators/SitemapGenerator.php	100.00% <0.00%> (ø)
...es/framework/src/Console/Commands/ServeCommand.php	100.00% <0.00%> (ø)
...rc/Framework/Features/Metadata/PageMetadataBag.php	100.00% <0.00%> (ø)
.../Framework/Features/Metadata/GlobalMetadataBag.php	100.00% <0.00%> (ø)
...ework/src/Console/Commands/BuildRssFeedCommand.php	100.00% <0.00%> (ø)
...packages/framework/src/Pages/Concerns/HydePage.php	100.00% <0.00%> (ø)
... and 133 more

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