Update architecture doc for IPAM, Prometheus, and README for features (#3814)

Update the IPAM description and add Prometheus integration in the
architecture doc.
Update the encryption description in README and add a link to the
Features section of the getting-started doc.

Signed-off-by: Jianjun Shen <shenj@vmware.com>

Author: jianjuns

README.md

@@ -98,12 +98,14 @@ Also check out [@ProjectAntrea](https://twitter.com/ProjectAntrea) on Twitter!
   analysis, flow inspection). It exposes Prometheus metrics and supports
   exporting network flow information which can be visualized in Kibana
   dashboards.
-* **Encryption**: Encryption of inter-Node Pod traffic with IPsec tunnels when
-  using an overlay Pod network.
+* **Encryption**: Encryption of inter-Node Pod traffic with IPsec or WireGuard
+  tunnels.
 * **Easy deployment**: Antrea is deployed by applying a single YAML manifest
   file.
 
-Refer to the [Changelogs](CHANGELOG/README.md) for a detailed list of features
+To explore more Antrea features and their usage, check the [Getting started](docs/getting-started.md#features)
+document and user guides in the [Antrea documentation folder](docs/). Refer to
+the [Changelogs](CHANGELOG/README.md) for a detailed list of features
 introduced for each version release.
 
 ## Adopters

docs/design/architecture.md

@@ -159,14 +159,14 @@ tunnels to other Nodes.
 
 <img src="../assets/node.svg.png" width="300" alt="Antrea Node Network">
 
-Each Node is assigned a single subnet, and all Pods on the Node get an IP from
-the subnet. Antrea leverages Kubernetes' `NodeIPAMController` for the Node
-subnet allocation, which sets the `podCIDR` field of the Kubernetes Node spec
-to the allocated subnet. Antrea Agent retrieves the subnets of Nodes from the
-`podCIDR` field. It reserves the first IP of the local Node's subnet to be the
-gateway IP and assigns it to the `antrea-gw0` port, and invokes the
-[host-local IPAM plugin](https://github.com/containernetworking/plugins/tree/master/plugins/ipam/host-local)
-to allocate IPs from the subnet to all local Pods. A local Pod is assigned an IP
+By default, Antrea leverages Kubernetes' `NodeIPAMController` to allocate a
+single subnet for each Kubernetes Node, and Antrea Agent on a Node allocates an
+IP for each Pod on the Node from the Node's subnet. `NodeIPAMController` sets
+the `podCIDR` field of the Kubernetes Node spec to the allocated subnet. Antrea
+Agent retrieves the subnets of Nodes from the `podCIDR` field. It reserves the
+first IP of the local Node's subnet to be the gateway IP and assigns it to the
+`antrea-gw0` port, and invokes the [host-local IPAM plugin](https://github.com/containernetworking/plugins/tree/master/plugins/ipam/host-local)
+to allocate IPs from the subnet to all Pods. A local Pod is assigned an IP
 when the CNI ADD command is received for that Pod.
 
 `NodeIPAMController` can run in `kube-controller-manager` context, or within
@@ -176,6 +176,10 @@ For every remote Node, Antrea Agent adds an OVS flow to send the traffic to that
 Node through the appropriate tunnel. The flow matches the packets' destination
 IP against each Node's subnet.
 
+In addition to Kubernetes NodeIPAM, Antrea also implements its own IPAM feature,
+which can allocate IPs for Pods from user-defined IP pools. For more
+information, please refer to the [Antrea IPAM documentation](../antrea-ipam.md).
+
 ### Traffic walk
 
 <img src="../assets/traffic_walk.svg.png" width="600" alt="Antrea Traffic Walk">
@@ -354,6 +358,14 @@ using IPFIX. The exported network flows can be visualized using Elastic Stack
 and Kibana dashboards. For more information, refer to the [network flow
 visibility document](../network-flow-visibility.md).
 
+### Prometheus integration
+
+Antrea supports exporting metrics to Prometheus. Both Antrea Controller and
+Antrea Agent implement the `/metrics` API endpoint on their API server to expose
+various metrics generated by Antrea components or 3rd party components used by
+Antrea. Prometheus can be configured to collect metrics from the API endpoints.
+For more information, please refer to the [Prometheus integration document](../prometheus-integration.md).
+
 ### Windows Node
 
 On a Windows Node, Antrea acts very much like it does on a Linux Node. Antrea