Add documentation about RLE-bitpack hybrid encoder

Since this is a parquet-specific encoder, it would be good to have a more complete description of the encoding/decoding, so that implementations have a easier time implementing it.

**Reporter**: [Jorge Leitão](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=jorgecarleitao) / @jorgecarleitao

_{**Note**: *This issue was originally created as [PARQUET-2021](https://issues.apache.org/jira/browse/PARQUET-2021). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

No file, test, or entry point is named. Start by reviewing the existing RLE-bitpack hybrid encoder material and the migration documentation linked in the issue, then document the encoding and decoding behavior in the appropriate Parquet format documentation. Done means implementations have a complete description they can use to implement the encoder and decoder.

Written by the indexing model from the issue text.