[docs] Migrate docs from AsciiDoc to Markdown (#2806)

* delete asciidoc files

* add migrated files

* Fix Read the Docs build

* fix links

* clean up cross-repo links

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Co-authored-by: Quentin Pradet <quentin.pradet@elastic.co>
Co-authored-by: Marci W <333176+marciw@users.noreply.github.com>

Author: 3 people