Fix plugin-list script (elastic#215648)

## Summary

A [recent PR](https://github.com/elastic/kibana/pull/212558/files) (3
weeks ago) migrated docs under `/docs` folder, from `.asciidoc` to
`.md`.

Unfortunately, some of these docs (well, at least one) were generated
automatically using a script:

`node scripts/build_plugin_list_docs.js` was updating the
`docs/developer/plugin-list.asciidoc`

As a result of the migration:

The CI pipeline step fails, and the list of plugins is no longer
updated.
Besides:
* The https://www.elastic.co/guide/en/kibana/current/plugin-list.html is
currently broken.
* This page is (or should be) built from
https://github.com/elastic/kibana/blob/main/docs/extend/plugin-list.md,
which is also broken.

They are broken cause some plugins' descriptions use the pipe `|`
character, which breaks the Markdown layout.

This PR updates the logic that generates the plugin-list, switching from
`.asciidoc` to `.md`, and escaping `\n` and `|` characters to allow for
better descriptions.

Author: gsoldevila

.buildkite/scripts/steps/checks/quick_checks.txt

@@ -2,6 +2,7 @@
 .buildkite/scripts/steps/checks/packages.sh
 .buildkite/scripts/steps/checks/bazel_packages.sh
 .buildkite/scripts/steps/checks/verify_notice.sh
+.buildkite/scripts/steps/checks/plugin_list_docs.sh
 .buildkite/scripts/steps/checks/event_log.sh
 .buildkite/scripts/steps/checks/telemetry.sh
 .buildkite/scripts/steps/checks/jest_configs.sh