Split up adoc files

[philwebb]

Our adoc files are quite large, it would be nice to split them up a bit.

[wilkinsona]

I've split up spring-boot-features.adoc so that we can get a feel for what the end result might be like: https://github.com/wilkinsona/spring-boot/tree/gh-23254.

[wilkinsona]

Automating the split is hampered a bit by some of our section ids. For example, boot-features-custom-log-levels and boot-features-custom-log-groups are both nested beneath boot-features-logging but this isn't reflected in their IDs. Ideally, they would be named boot-features-logging-custom-log-levels and boot-features-logging-custom-log-groups. Unfortunately, correcting the IDs to follow our conventions will break existing links to those specific sections.

[wilkinsona]

We also have some sections with no ID so they're using Asciidoctor's default which also doesn't match our conventions. The last four sections in 2.1.x's documentation-overview.adoc for example.

[philwebb]

We might be able to add multiple anchors. I did that with "External Application Properties" in spring-boot-features.adoc.