Add configuration management topics to the Beats docs

[dedemorton]

Edits and adds content to the Beats to cover configuration management.

You can see the built docs on Firebase here: https://beatsmanagement-b2e5c.firebaseapp.com/configuration-central-management.html

A few notes:

• Asciidoc can't seem to handle images added with includes, so I've had to copy the referenced images into the relevant image folders for each Beat. I'll investigate a better way of doing this, but I've gone down that rabbit hole before with no success.

• I've used screen captures instead of conceptual diagrams, but I'd like to replace the screen captures at some point because they are likely to get out-of-date quickly, and I don't think screen captures are necessary to explain the concepts.

• I've flagged the content with the x-pack badge, but I don't mention that a gold license is required because we've been told in the past to avoid mentioning specific licenses in the docs. Now that x-pack isn't being used, though, 🤷‍♀️ .

• Our convention is to use uppercase instead of < > for variable values shown in syntax descriptions. we might change that, but for now, I've changed the command example to follow our contention.

• Did a little restructuring, but we can change it later if it doens't work. I decided to pop the section up in the TOC because I felt that it was too hidden under the configuring topic. I will add links from those topics to point to the central management docs. I also broke the topic into several sections because I wanted to add a place where we can put the API docs later. Also, I wanted the topic about enrolling beats to have a higher SEO ranking.

• I also plan to add a page to the Kibana docs to explain basic operations in the UI and point users to these docs.

[exekias]

I would go for commenting it out until we actually have it

[exekias]

Left a minor comment, OK to merge & backport as it is to get it before the release