Add WebAssembly page. (#6738)

* Add WebAssembly page.

Signed-off-by: John Plevyak <jplevyak@gmail.com>

* Address comments.

Signed-off-by: John Plevyak <jplevyak@gmail.com>

* Update content/en/docs/concepts/wasm/index.md

Co-Authored-By: mandarjog <mandarjog@gmail.com>

* proxy-Wasm -> Proxy-Wasm.

* Style and ecosystem.

* Fix links.

* Extensibility

* Fix typo.

* Add missing word to the spelling dictionary.

* Remove trailing whitespace.

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

Co-authored-by: Piotr Sikora <piotrsikora@google.com>
Co-authored-by: mandarjog <mandarjog@gmail.com>

.spelling

@@ -531,6 +531,7 @@ runtimes
 sa
 Salesforce
 sandboxed
+sandboxing
 sayin
 schemas
 SDKs

content/en/docs/concepts/wasm/index.md

@@ -0,0 +1,56 @@
+---
+title: Extensibility
+description: Describes Istio's WebAssembly Plugin system.
+weight: 25
+keywords: [wasm,webassembly,emscripten,extension,plugin,filter]
+---
+
+WebAssembly is a sandboxing technology which can be used to extend the Istio proxy (Envoy).  The Proxy-Wasm sandbox API replaces Mixer as the primary extension mechanism in Istio. Istio 1.6 will provide a uniform configuration API for Proxy-Wasm plugins.
+
+WebAssembly sandbox goals:
+
+- **Efficiency** - An extension adds low latency, CPU, and memory overhead.
+- **Function** - An extension can enforce policy, collect telemetry, and perform payload mutations.
+- **Isolation** - A programming error or crash in one plugin does affect other plugins.
+- **Configuration** - The plugins are configured using an API that is consistent with other Istio APIs. An extension can be configured dynamically.
+- **Operator** - An extension can be canaried and deployed as log-only, fail-open or fail-close.
+- **Extension developer** - The plugin can be written in several programming languages.
+
+This [video talk](https://youtu.be/XdWmm_mtVXI) is an introduction about architecture of WebAssembly integration.
+
+## High-level architecture
+
+Istio extensions (Proxy-Wasm plugins) have several components:
+
+- **Filter Service Provider Interface (SPI)** for building Proxy-Wasm plugins for filters.
+- **Sandbox** V8 Wasm Runtime embedded in Envoy.
+- **Host APIs** for headers, trailers and metadata.
+- **Call out APIs** for gRPC and HTTP calls.
+- **Stats and Logging APIs** for metrics and monitoring.
+
+{{< image width="80%" link="./extending.svg" caption="Extending Istio/Envoy" >}}
+
+## Example
+
+An example C++ Proxy-Wasm plugin for a filter can be found
+[here](https://github.com/envoyproxy/envoy-wasm/tree/19b9fd9a22e27fcadf61a06bf6aac03b735418e6/examples/wasm).
+
+To implement a Proxy-Wasm plugin for a filter:
+
+- Implement a [root context class](https://github.com/envoyproxy/envoy-wasm/blob/e8bf3ab26069a387f47a483d619221a0c482cd13/examples/wasm/envoy_filter_http_wasm_example.cc#L7) which inherits [base root context class](https://github.com/envoyproxy/envoy-wasm/blob/e8bf3ab26069a387f47a483d619221a0c482cd13/api/wasm/cpp/proxy_wasm_impl.h#L288)
+- Implement a [stream context class](https://github.com/envoyproxy/envoy-wasm/blob/e8bf3ab26069a387f47a483d619221a0c482cd13/examples/wasm/envoy_filter_http_wasm_example.cc#L14) which inherits the [base context class](https://github.com/envoyproxy/envoy-wasm/blob/e8bf3ab26069a387f47a483d619221a0c482cd13/api/wasm/cpp/proxy_wasm_impl.h#L314).
+- Override [context API](https://github.com/envoyproxy/envoy-wasm/blob/e8bf3ab26069a387f47a483d619221a0c482cd13/examples/wasm/envoy_filter_http_wasm_example.cc#L14) methods to handle corresponding initialization and stream events from host.
+- [Register](https://github.com/envoyproxy/envoy-wasm/blob/e8bf3ab26069a387f47a483d619221a0c482cd13/examples/wasm/envoy_filter_http_wasm_example.cc#L26) the root context and stream context.
+
+## SDK
+
+A detailed description of the C++ SDK can be found [here](https://github.com/proxy-wasm/proxy-wasm-cpp-sdk/tree/a30aaeedf30cc1545318505574c7fb3bb8d8c243/docs/wasm_filter.md).
+
+## Ecosystem
+
+- [Proxy-Wasm ABI specification](https://github.com/proxy-wasm/spec)
+- [Proxy-Wasm C++ SDK](https://github.com/proxy-wasm/proxy-wasm-cpp-sdk)
+- [Proxy-Wasm Rust SDK](https://github.com/proxy-wasm/proxy-wasm-rust-sdk)
+- [Proxy-Wasm AssemblyScript SDK](https://github.com/solo-io/proxy-runtime)
+- [WebAssembly Hub](https://docs.solo.io/web-assembly-hub/latest/tutorial_code/)
+- [WebAssembly Extensions For Network Proxies (video)](https://www.youtube.com/watch?v=OIUPf8m7CGA)