Add performance tuning related documentation for 4.x.x documentation

[ashera96]

Description:

We need to fact-check the documentation [1]. Refer to the following section under Scaling Gateways and Traffic Managers:

Based on the performance numbers attained at the WSO2 Labs, the recommended ratio is 10 Gateways to 1 Traffic Manager (10:1, one Traffic Manager is tested to have successfully handled the load of 10 Gateways, one Gateway handling the load  of ~5000 TPS).

This section implies that, if 1 GW handles 5k TPS, with 10 GWs, we should be able to handle 50k TPS. However, the number that we have tested is 20k TPS. Therefore, we need to fact-check the info in this section.

Also, the section Clustering Gateways and Key Managers with key caching (refer to [2]) should be removed as it is no longer applicable.

Looks like the documentation [3] was removed from 4.x onwards. We have to consider adding it back to the 4.x docs as we no longer have any documentation related to performance tuning. More specifically, on tuning data-agent parameters.

[transport.binary.agent]
queue_size
batch_size
core_pool_size
max_pool_size
max_transport_pool_size
secure_max_transport_pool_size

[1] https://apim.docs.wso2.com/en/latest/deploy-and-publish/deploy-on-gateway/api-gateway/scaling-the-gateway/#scaling-gateways-and-traffic-managers
[2] https://apim.docs.wso2.com/en/latest/deploy-and-publish/deploy-on-gateway/api-gateway/scaling-the-gateway/#clustering-gateways-and-key-managers-with-key-caching
[3] https://apim.docs.wso2.com/en/3.2.0/install-and-setup/setup/deployment-best-practices/tuning-performance/#tuning-data-agent-parameters

Affected Product Versions:
APIM 4.0.0
APIM 4.1.0
APIM 4.2.0
APIM 4.3.0
APIM 4.4.0