Update Zephyr document, add esp32c3 and particle_argon support(#920)

Update the Zephyr document to provide more detailed instructions, and add info
about espressif toolchain too.

Add ESP32C3 (RISC-V) and Particle Argon boards support to the zephyr platform
sample. More boards are possible, but the script doesn't scale well and is to be
improved in future.

Add Dockerfile to kickstart a Zephyr development environment as it can be rather
confusing for new users.

product-mini/platforms/zephyr/simple/README_docker.md

@@ -0,0 +1,25 @@
+# Build with Docker
+
+To have a quicker start, a Docker container of the Zephyr setup can be generated.
+
+## Build Docker container
+
+``` Bash
+docker build --build-arg DOCKER_UID=$(id -u) . -t wamr-zephyr
+```
+
+## Run Docker container to build images
+
+Enter the docker container (maps the toplevel wasm-micro-runtime repo as volume):
+
+``` Bash
+docker run -ti -v $PWD/../../../..:/home/wamr/source --device=/dev/ttyUSB0 wamr-zephyr
+```
+
+Adopt the device or remove if not needed.
+
+And then in the docker container:
+
+``` Bash
+./build_and_run.sh esp32c3
+```