[DOCS] Release highlights have broken heading links & no code highlighting

[debadair]

The generated release highlights have PR links in the headings. These links conflict with the anchor links added by the doc build and don't work as intended. In addition, the code snippets use Markdown-style code blocks instead of Asciidoc, so there's no code highlighting.

To fix the link issue, let's move the PR links to the end of the descriptions, similar to the way we handle the GH links in the Release notes. For example:

[discrete]
[[ingest_performance_improvement]]
=== Ingest performance improvement
We have improved the pipeline execution logic for pipelines with processors that are synchronous
by avoiding (deep) recursion.
On our nightly benchmark that simulates a Logging use-case, this resulted in a 10% reduction of
CPU time spent on ingest pipelines and a 3% overall ingestion speedup. 

{es-pull}84250[#84250]

To add code highlighting to the snippets, they need to be formatted as Asciidoc code blocks. For example:

[source,yaml]
----
{
  "metrics.time" : 10,
  "metrics.time.min" : 1,
  "metrics.time.max" : 500
}
----

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[debadair]

Updated to recommend that the link be on a new line, as there are some multi-paragraph highlights.

[debadair]

Updated to recommend that the link be on a new line, as there are some multi-paragraph highlights.