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ARROW-8275: [Python] Update Feather documentation for V2, Python IPC API cleanups / deprecations

This splits out the Feather documentation into its own section and explains the V2 changes (support for all Arrow types and compression).

This adds a FutureWarning to most of the `pyarrow.ipc` functions that are in the `pyarrow.*` namespace. Since these functions may cause confusion amongst non-advanced users, it's most clear what they are when accessed via the `pyarrow.ipc` namespace, for example `pa.ipc.read_schema`. This is consistent with the prior deprecation of `pa.open_stream` and `pa.open_file`.

Also disables failure-on-warning when using 'make html' to build Sphinx docs. Fix various Sphinx warnings. I had to pin Sphinx 2.4.4 on account of the newly released Sphinx 3.0.0 not being compatible with our Sphinx project, see ARROW-8340.

Closes #6843 from wesm/ARROW-8275

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>