[docs] Migrate docs from AsciiDoc to Markdown

[colleenmcginnis]

Migrate docs from AsciiDoc to Markdown. The preview can be built after #1115 is merged.

Note: Ideally we would also remove the AsciiDoc files from the main branch. In other repos the docs team has been creating a temporary branch, do-not-delete_legacy-docs, to host legacy AsciiDoc content until we can turn on our new Markdown docs when 9.0 launches. However, no one on the docs team has permissions to create a new branch in this repo. Would someone be able to assist us with this?

cc @leemthompo @elastic/search-experiences-team (I can't add reviewers)

[github-actions]

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[leemthompo]

@colleenmcginnis did you want to target the do-not-delete_legacy-docs branch for this one?

[colleenmcginnis]

@colleenmcginnis did you want to target the do-not-delete_legacy-docs branch for this one?

No. The do-not-delete_legacy-docs will be used by the AsciiDoc build system (after elastic/docs#3181 is merged) so we don't need the Markdown files on that branch.

[leemthompo]

FYI I opened link fix PR in logstash repo