Docs: Add rmem_default & wmem_default to validator sysctl tuning

[therainisme]

Problem

The current system tuning documentation for Agave validators is missing recommended sysctl settings for net.core.rmem_default and net.core.wmem_default.

For example, the following warnings were observed when running an RPC node with the current documentation's tuning guide:

[2025-06-05T04:01:41.538206515Z WARN  solana_core::system_monitor_service]   net.core.rmem_default: recommended=134217728, current=212992 too small
OS network limit test failed. See: https://docs.solanalabs.com/operations/guides/validator-start#system-tuning

[2025-06-05T04:05:48.306590702Z WARN  solana_core::system_monitor_service]   net.core.wmem_default: recommended=134217728, current=212992 too small
OS network limit test failed. See: https://docs.solanalabs.com/operations/guides/validator-start#system-tuning

Summary of Changes

This PR updates the "System Tuning -> Linux -> Optimize sysctl knobs" section of the Agave validator setup documentation.

The following recommended sysctl settings have been added to the guide, for inclusion in /etc/sysctl.d/21-agave-validator.conf:

net.core.rmem_default = 134217728
net.core.wmem_default = 134217728