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ARROW-6887: [Java] Create prose documentation for using ValueVectors

Related to [ARROW-6887](https://issues.apache.org/jira/browse/ARROW-6887).
We should create documentation (in restructured text) for the library that demonstrates:

1.  Basic construction of ValueVectors.  Highlighting:
    * ValueVector lifecycle
    * Reading by rows using Readers (mentioning that it is not as efficient as direct access).
    * Populating with Writers
2.  Reading and writing IPC stream format and file formats.

Closes #5723 from tianchen92/ARROW-6887 and squashes the following commits:

07f10c9c2 <tianchen> fix some syntax problems and add dictionary examples
5625ed25c <tianchen> fix build fail
b04661295 <tianchen> fix style
ec8e2fc59 <tianchen> update docs and fix build fail
b6001a289 <tianchen> fix style
0f00fee26 <tianchen> update docs
35392f908 <tianchen> ARROW-6887:  Create prose documentation for using ValueVectors

Authored-by: tianchen <niki.lj@alibaba-inc.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>