[SPARK-30481][DOCS][FOLLOWUP] Document event log compaction into new section of monitoring.md
[SPARK-30481][DOCS][FOLLOWUP] Document event log compaction into new section of monitoring.md

### What changes were proposed in this pull request?

This is a FOLLOW-UP PR for review comment on #27208 : https://github.com/apache/spark/pull/27208#pullrequestreview-347451714

This PR documents a new feature `Eventlog Compaction` into the new section of `monitoring.md`, as it only has one configuration on the SHS side and it's hard to explain everything on the description on the single configuration.

### Why are the changes needed?

Event log compaction lacks the documentation for what it is and how it helps. This PR will explain it.

### Does this PR introduce any user-facing change?

No.

### How was this patch tested?

Built docs via jekyll.

> change on the new section

<img width="951" alt="Screen Shot 2020-02-16 at 2 23 18 PM" src="https://user-images.githubusercontent.com/1317309/74599587-eb9efa80-50c7-11ea-942c-f7744268e40b.png">

> change on the table

<img width="1126" alt="Screen Shot 2020-01-30 at 5 08 12 PM" src="https://user-images.githubusercontent.com/1317309/73431190-2e9c6680-4383-11ea-8ce0-815f10917ddd.png">

Closes #27398 from HeartSaVioR/SPARK-30481-FOLLOWUP-document-new-feature.

Authored-by: Jungtaek Lim (HeartSaVioR) <6dfb5872701d21af1c4dd7add991df02fba61fbf@gmail.com>
Signed-off-by: Dongjoon Hyun <9b02cb7e37158797a8e22c559f7c5f5ecd28ea57@apple.com>

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