[FLINK-18804][docs][config] Skip file generation for empty tables

[zentol]

The ConfigDocsGenerator no longer creates files that do not contain any options.

This can happen if some class matches the naming pattern we use, but doesn't actually contain options, or if config groups aren't matching any options.

This PR also includes some cleanup hotfixes; sorting entries and pruning unused files.

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Last check on commit 63bfa99 (Mon Aug 03 11:49:16 UTC 2020)

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[XComp]

No remarks on the code. 👍 Building the docs resulted in a modified version of docs/_includes/generated/yarn_config_configuration.html, though. This might be caused by me having a more recent snapshot installed in my local repository for the YARN configuration.