Restructure reference docs

[swallez]

Restructure the reference documentation to have a file hierarchy that follows the documentation structure.

[swallez]

Errors reported in the doc-build/preview/build check will be fixed by elastic/docs-content#1071

[github-actions]

The backport to 9.0 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-9.0 9.0
# Navigate to the new working tree
cd .worktrees/backport-9.0
# Create a new branch
git switch --create backport-988-to-9.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 c8908c84c7f8e763686e3bc6163584a1e9cd70a6
# Push it to GitHub
git push --set-upstream origin backport-988-to-9.0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.0

Then, create a pull request where the base branch is 9.0 and the compare/head branch is backport-988-to-9.0.

[marciw]

@swallez @l-trotta I only just realized this was merged in with docs build errors 😦 (https://github.com/elastic/elasticsearch-java/actions/runs/14374038419/job/40302335689)

... I'm working on fixing this now, but for future reference, this kind of restructuring needs a redirects.yml file: https://elastic.github.io/docs-builder/contribute/redirects/

[marciw]

... I'm working on fixing this now, but for future reference, this kind of restructuring needs a redirects.yml file: https://elastic.github.io/docs-builder/contribute/redirects/

(link was wrong, fixed now)

[l-trotta]

sorry @marciw, we were convinced the build errors were because of the missing pr on the docs side, not the opposite

[marciw]

sorry @marciw, we were convinced the build errors were because of the missing pr on the docs side, not the opposite

thanks @l-trotta -- no worries. When files move around, it creates a chicken-and-egg issue with inbound link validation in the docs build. So a redirects.yml file is needed: https://elastic.github.io/docs-builder/contribute/redirects/
(see also https://elastic.github.io/docs-builder/contribute/move/)

For this one, I opened #997