[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

filebeat/docs/inputs/input-filestream-reader-options.asciidoc の ndjson セクションから始め、GitHub でのレンダリングを、リンク先の filebeat-input-filestream ページにある Elastic Docs 版と比較します。parser と parser-option のフォーマットを担っているドキュメント用 CSS を追跡します。完了条件は、parser のオプションが太字斜体の等幅フォントで表示され、parser は太字の等幅フォントのままになることです。

索引モデルが issue の本文から書いたものです。