[Docs] modify CSS to show "parser options" in italic in the Elastic Docs, similar to Github

In our documentation in Github at https://github.com/elastic/beats/blob/8.5/filebeat/docs/inputs/input-filestream-reader-options.asciidoc#ndjson. we can see that the parser (e.g., `ndjson`) is formatted in a **bold mono** font.

The parser options (e.g., `target`) are formatted as a **_bold italic mono_** font.

This makes it easy to distinguish between the Parser and the Parser options.

Looking at the resulting document in our docs at https://www.elastic.co/guide/en/beats/filebeat/current/filebeat-input-filestream.html#_ndjson, we can see that the Parser and the Parser Options have the same formatting. They are both bold mono font with the parser options missing the italic formatting.

This makes it harder to read the official document compared to the GitHub document. A customer complained and asked if we can make the Parser Options in our doc italic, too, for easy readability.

GitHub (good formatting):

Elastic Docs (bad formatting):

Originally filed at https://github.com/elastic/website-requests/issues/840

Start with filebeat/docs/inputs/input-filestream-reader-options.asciidoc at the ndjson section and compare its GitHub rendering with the Elastic Docs version at the linked filebeat-input-filestream page. Trace the documentation CSS responsible for parser and parser-option formatting; done means parser options render bold italic mono while parsers remain bold mono.

Written by the indexing model from the issue text.