[DOCS] Reorganizing docs directory structure

[ebeahan]

Summary

Reorganize the docs folder to match the organization on the rendered docs site.

The new directory structure can be browsed on my branch here.

Background

Until now, the ECS .asciidoc files were a collection of files under the top-level docs directory (excluding the more recently added docs/usage). This structure was becoming a bit disorganized, and it was hard to understand the docs hierarchy without opening individual files and "to connect the dots" looking at include:: statements.

Changes

• New subdirectories:

  • docs/using-ecs
  • docs/fields
  • docs/migrating
  • docs/additional-info

• The docs/index.asciidoc remains the top-level doc file for ECS.

• docs/using-ecs, docs/migrating, and docs/additional-info now contain an index.asciidoc for that section's top-level doc page

• Rename specific .asciidoc files to remove now redundant prefixes (for example, using-).

• Small changes to the asciidoc generator script to accommodate the new directory structure.

• Deleted the unused docs/logstash.asciidoc.disabled file. Easy to add a file in the future if this is something we decide to pursue.

Non-changes

• The way the doc builds generates the rendered docs site, URLs will not be changed by reorganizing the files in the repo.
• The site will display the docs the same as before these changes. Again, this is just an administrative change that tidies up the docs.

[ebeahan]

@elasticmachine run elasticsearch-ci/docs