Review the structure and headings of Beats configuration docs

[vishaangelova]

Review the structure of the docs under the Configure section of the different Beats, and fix the heading issues (especially for the inputs and processors).

[vishaangelova]

The way to handle duplicate headings is discussed in elastic/docs-builder#490. The current issue will focus on reviewing the structure of the docs.

For example, certain configuration options can be grouped together under a parent heading (in Azure Blob Storage Input, the content-type and encoding headings should be nested under Custom properties).