[Python][C++] Document how to write _metadata, _common_metadata files with Parquet datasets

[asfimport]

This is not mentioned in great detail in

http://arrow.apache.org/docs/python/parquet.html

Reporter: Wes McKinney / @wesm
Assignee: Joris Van den Bossche / @jorisvandenbossche

Related issues:

• [Python][Dataset] Detect and use _metadata file in a list of file paths (relates to)

PRs and other links:

• GitHub Pull Request #7348

_{Note: This issue was originally created as ARROW-3154. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
This issue may be subsumed with the broader migration to a C++-based implementation

cc @bkietz @fsaintjacques

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
Issue resolved by pull request 7348
#7348