[docs] Migrate docs from AsciiDoc to Markdown (elastic#212558)

Migrate docs from AsciiDoc to Markdown. The preview can be built after
elastic#212557 is merged.

@florent-leborgne please tag reviewers, add the appropriate label(s),
and take this out of draft when you're ready.

Note: More files are deleted than added here because the content from
some files was moved to
[elastic/docs-content](https://github.com/elastic/docs-content).

**What has moved to
[elastic/docs-content](https://github.com/elastic/docs-content)?**

Public-facing narrative and conceptual docs have moved. Most can now be
found under the following directories in the new docs:
- explore-analyze: Discover, Dashboards, Visualizations, Reporting,
Alerting, dev tools...
- deploy-manage: Stack management (Spaces, user management, remote
clusters...)
- troubleshooting: .... troubleshooting pages

**What is staying in the Kibana repo?**

- Reference content (= anything that is or could be auto-generated):
Settings, syntax references
- Release notes
- Developer guide

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Co-authored-by: Florent Le Borgne <[email protected]>

Author: colleenmcginnis