Document Logstash's file descriptors usage, limits and sizing guidance

[jsvd]

This commit adds documentation about the consumption of file descriptor usage in Logstash, from plugins to PQ/DLQ.
Then warns about the limits set by the Operating Systems, how to consult and raise them.
Finally, provides recommendations on how to set a reasonable limit for production environments.

fixes #18583

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)
• /run exhaustive tests : Run the exhaustive tests Buildkite pipeline.

[mergify]

This pull request does not have a backport label. Could you fix it @jsvd? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[github-actions]

🔍 Preview links for changed docs

• docs/reference/file-descriptors.md
• docs/reference/tips-best-practices.md