Backport of #51867.
Tweak the documentation around configuring the heap size when using
Docker, to state that:
- using `ES_JAVA_OPTS` is the preferred method
- Any `ES_JAVA_OPTS` overrides the defaults in `jvm.options`
- It's possible to bind-mount a custom `jvm.options`
This commit reorganizes some of the content in the configuring
Elasticsearch section of the docs. The changes are:
- move JVM options out of system configuration into configuring
Elasticsearch
- move JVM options to its own page of the docs
- move configuring the heap to important Elasticsearch settings
- move configuring the heap to its own page of the docs
- move all important settings to individual pages in the docs
- remove bootstrap.memory_lock from important settings, this is covered
in the swap section of system configuration
Relates #27755