[24150] Add send rate limiter configuration for PDPServer

[raulojeda22]

Description

• Document discoveryServer_send_period / <serverSendPeriod> in the Discovery Server section.
• Add new subsection explaining the send rate limiter and its trade-offs.
• Rewrite the handshake tuning section to clarify the dual role of discoveryServer_client_syncperiod (client ping period vs. server routine fallback re-check).
• Add <serverSendPeriod> to the XML configuration reference table with a note that it is a Pro-only feature.

Contributor Checklist

• Commit messages follow the project guidelines.
• NO Code snippets related to the added documentation have been provided.
• Documentation tests pass locally.
• The Pro version badge has been added if the documented feature is exclusive to Fast DDS Pro.
• N/A Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• CI passes without warnings or errors.