Modify Makefile for docs

[snleee]

In order to avoid including "_build" directory for source code bundle,
this pr modifies the behavior of "make clean" to remove build directory.

[codecov-io]

Codecov Report

Merging #3771 into master will increase coverage by 0.13%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #3771      +/-   ##
===========================================
+ Coverage     67.17%   67.3%   +0.13%     
  Complexity        4       4              
===========================================
  Files          1027    1027              
  Lines         50799   50799              
  Branches       7091    7091              
===========================================
+ Hits          34123   34192      +69     
+ Misses        14350   14269      -81     
- Partials       2326    2338      +12

Impacted Files	Coverage Δ	Complexity Δ
...core/operator/dociditerators/AndDocIdIterator.java	51.72% <0%> (-31.04%)	0% <0%> (ø)
...pinot/core/operator/docidsets/OrBlockDocIdSet.java	84.9% <0%> (-13.21%)	0% <0%> (ø)
...pache/pinot/core/util/SortedRangeIntersection.java	83.82% <0%> (-7.36%)	0% <0%> (ø)
.../impl/dictionary/LongOffHeapMutableDictionary.java	87.27% <0%> (-5.46%)	0% <0%> (ø)
...r/filter/predicate/PredicateEvaluatorProvider.java	64% <0%> (-4%)	0% <0%> (ø)
...mpl/dictionary/DoubleOffHeapMutableDictionary.java	69.09% <0%> (-3.64%)	0% <0%> (ø)
...e/operator/dociditerators/SVScanDocIdIterator.java	67.85% <0%> (-3.58%)	0% <0%> (ø)
...e/operator/dociditerators/BitmapDocIdIterator.java	60.71% <0%> (-3.58%)	0% <0%> (ø)
...inot/core/operator/docidsets/AndBlockDocIdSet.java	55.55% <0%> (-2.09%)	0% <0%> (ø)
...ata/manager/realtime/RealtimeTableDataManager.java	41.17% <0%> (-1.48%)	0% <0%> (ø)
... and 26 more

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[mcvsubbu]

AFAIK this file was auto-generated, but I am not sure. Maybe @jfim can confirm. Otherwise looks ok