[DOCS] Rename monitoring collection from internal to legacy

[lcawl]

Related to elastic/logstash#11843, elastic/kibana#64661, https://github.com/elastic/elastic-stack-testing/pull/549

This PR synchs Elasticsearch monitoring documentation with terminology changes across the stack (i.e. refer to "internal" collection as "legacy" collection). It also groups all of the pages related to the legacy collection methods under a "Legacy collection methods" section in the navigation.

Preview:
http://elasticsearch_56395.docs-preview.app.elstc.co/diff
For example: http://elasticsearch_56395.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/collecting-monitoring-data.html

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Monitoring)

[jakelandis]

@lcawl - apologizes for the delay this slipped off my radar. The text changes look good, but have a bit of a concern about rename.

@ycombinator @chrisronline - I worry that changing this to legacy in our public docs is a bit premature. Given that cloud uses these collectors and if you want to export monitoring data from cloud to on prem or to another cloud instance these collectors are the only way. I think we need to be careful calling something "legacy" without an alternative across deployment scenarios.

[chrisronline]

cc @ravikesarwani

[chrisronline]

@jakelandis I'm going to assume that you aren't going to be comfortable marking internal/legacy collectors as deprecated in the docs either? We're planning on doing that across the board in 7.9 but I want to make sure we think through your concerns before committing to anything on the Elasticsearch side.

[jakelandis]

I'm going to assume that you aren't going to be comfortable marking internal/legacy collectors as deprecated in the docs either?

@chrisronline I checked out the documentation for ESS/ECE where monitoring is implemented as abstraction through the platform (i.e. not requiring the user to setting settings these directly) and their doc reflects that. So I don't have strong objections, but did want to raise the point. I will defer to whatever decision is made, (which sounds like it already been made).

@lcawl - LGTM