[docs] Migrate docs from AsciiDoc to Markdown (#4479)

* delete asciidoc files

* add migrated files

* clean up cross-repo links

* add docs-content to cross_links

* update lambda page

* fix external links

* some clean up

* update release notes

* delete asciidoc changelog

* update references to CHANGELOG.asciidoc

* address feedback from @trentm

* remove unused subs

* move images

* fix more image paths

Author: colleenmcginnis

.github/pull_request_template.md

@@ -16,5 +16,5 @@ https://help.github.com/articles/closing-issues-using-keywords/
 - [ ] Add tests
 - [ ] Update TypeScript typings
 - [ ] Update documentation
-- [ ] Add CHANGELOG.asciidoc entry
+- [ ] Add release notes to `docs/release-notes/index.md`
 - [ ] Commit message follows [commit guidelines](https://github.com/elastic/apm-agent-nodejs/blob/main/CONTRIBUTING.md#commit-message-guidelines)