HBASE-22986 Add documentation for hbtop

[brfrn169]

No description provided.

[Apache-HBase]

🎊 +1 overall

Vote	Subsystem	Runtime	Comment
💙	reexec	1m 13s	Docker mode activated.
		_ Prechecks _
💚	dupname	0m 1s	No case conflicting files found.
💙	markdownlint	0m 1s	markdownlint was not available.
💚	@author	0m 1s	The patch does not contain any @author tags.
		_ master Compile Tests _
💙	mvndep	1m 5s	Maven dependency ordering for branch
💚	mvninstall	5m 11s	master passed
💙	refguide	5m 32s	branch has no errors when building the reference guide. See footer for rendered docs, which you should manually inspect.
		_ Patch Compile Tests _
💙	mvndep	0m 14s	Maven dependency ordering for patch
💚	mvninstall	4m 55s	the patch passed
💚	whitespace	0m 0s	The patch has no whitespace issues.
💙	refguide	5m 38s	patch has no errors when building the reference guide. See footer for rendered docs, which you should manually inspect.
		_ Other Tests _
💚	asflicense	0m 31s	The patch does not generate ASF License warnings.
		25m 47s

Subsystem	Report/Notes
Docker	Client=19.03.2 Server=19.03.2 base: https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-693/1/artifact/out/Dockerfile
GITHUB PR	#693
Optional Tests	dupname asflicense markdownlint refguide
uname	Linux 88f2c642d1e9 4.15.0-65-generic #74-Ubuntu SMP Tue Sep 17 17:06:04 UTC 2019 x86_64 GNU/Linux
Build tool	maven
Personality	/home/jenkins/jenkins-slave/workspace/HBase-PreCommit-GitHub-PR_PR-693/out/precommit/personality/provided.sh
git revision	master / 95c9911
refguide	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-693/1/artifact/out/branch-site/book.html
refguide	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-693/1/artifact/out/patch-site/book.html
Max. process+thread count	96 (vs. ulimit of 10000)
modules	C: hbase-hbtop . U: .
Console output	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-693/1/console
versions	git=2.11.0 maven=2018-06-17T18:33:14Z)
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[brfrn169]

Thank you for reviewing this! @petersomogyi