[apm docs consolidation] Delete moved files, update README

[colleenmcginnis]

Related to https://github.com/elastic/obs-docs-projects/issues/216

This deletes all docs files that have been moved to the elastic/observability-docs repo's docs/en/observability/apm/ directory. It also updates the README to reflect the changes made throughout the effort to consolidate APM docs in the Observability guide.

The docs-related files that will remain in this repo are:

• The changelog page listing all release notes is in CHANGELOG.asciidoc.
• Each minor version's release notes are documented in individual files in the changelogs/ directory.
• A list of all breaking changes are documented in changelogs/all-breaking-changes.asciidoc.
• Sample data sets that are injected into the docs are in the docs/data/ directory.
• Specifications that are injected into the docs are in the docs/spec/ directory.

Note: I also left one image that is used in the dev_docs/ARCHITECTURE.md file.

cc @bmorelli25

[mergify]

This pull request does not have a backport label. Could you fix it @colleenmcginnis? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[colleenmcginnis]

@bmorelli25 can you take a look at the README before I open PRs for other branches?

[bmorelli25]

🥲 bittersweet. Goodbye, APM Server repo.

[mergify]

This pull request is now in conflicts. Could you fix it @colleenmcginnis? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b apm-docs-consolidation-delete-files upstream/apm-docs-consolidation-delete-files
git merge upstream/main
git push upstream apm-docs-consolidation-delete-files