Add a documentation page for inter-zone traffic

[mariomac]

Points to cover:

• How to add src/dst zone attributes to current network metrics
• Why and how to add the new inter_zone_bytes metric
• Some useful PromQL queries. For example:

Interzone traffic:
rate(beyla_network_inter_zone_bytes_total[5m])

Same zone traffic:
rate(beyla_network_inter_zone_bytes_total[5m]) - rate(beyla_network_flow_bytes_total[5m])

% of interzone traffic vs total traffic:
rate(beyla_network_inter_zone_bytes_total[5m]) / rate(beyla_network_flow_bytes_total[5m])

% of same zone traffic vs total traffic:
(rate(beyla_network_flow_bytes_total[5m]) - rate(beyla_network_inter_zone_bytes_total[5m])) / rate(beyla_network_flow_bytes_total[5m])