Break-up Logstash docs + Add Centralized Pipeline Management documentation

[TimBosman]

I did three things in this PR:

• Go from one asciidoc file for Logstash documentation to a asciidoc file for every page
• Replaced some product names with attributes (e.g. Logstash -> {ls})
• Added a documentation page for Centralized Pipeline Management

Only thing I'm not completely sure the following section is recommended:

xpack.management.elasticsearch.username: "${PRODUCTION_ES_USER}"
xpack.management.elasticsearch.password: "${PRODUCTION_ES_PASSWORD}"

If I recall correctly this will setup a connection with the elastic user, which would be an escalation of permissions.

EDIT: It won't setup using the elastic user but with a different user (See the docs). This could still be an escalation of permissions.

Related issues:

• [Doc] Add central pipeline management example to Logstash #8462
• Doc: Break up source file for Logstash ECK docs into smaller files  #7616

[pebrc]

The documentation content is being moved to https://github.com/elastic/docs-content starting with the next release ECK 3.0. I wonder if t would make sense to make these changes there instead.