{Doc} Update cffi troubleshooting, extension authoring and onboarding guide (#14801)

* update for docker doc

* doc update

doc/extensions/authoring.md

@@ -120,7 +120,7 @@ See [Extension Metadata](metadata.md) for more information.
 ### Limit dependencies in setup.py
 
 - Before adding a dependency to your setup.py, check that it's not already available in [azure-cli-core setup.py](https://github.com/Azure/azure-cli/blob/master/src/azure-cli-core/setup.py).
-- Azure SDK or Azure Management SDK dependencies may be overridden by the versions installed as requirements of azure-cli-core. If you use any, test carefully, gracefully handle API changes, and be prepared to release updates. You might also consider rebasing the libraries under a different namespace (besides `azure`) to avoid conflicting with core CLI functionality.
+- Azure SDK or Azure Management SDK dependencies may be overridden by the versions installed as requirements of azure-cli-core. If you use any, test carefully, gracefully handle API changes, and be prepared to release updates. You might also consider rebasing the libraries under a different namespace (besides `azure`) to avoid conflicting with core CLI functionality. You can use [autorest](https://github.com/azure/autorest) to generate your SDK into a package that isn't under the `azure` directory.
 
 ### How do I know I'm using my dev extension(s)?
 

doc/install_troubleshooting.md

@@ -144,11 +144,15 @@ If you get errors on installation on **OS X**, upgrade pip by typing:
     pip install --upgrade --force-reinstall pip
 ```
 
-If you get errors on installation on **Fedora** such as `No module named '_cffi_backend'`,
+If you get errors on installation on **Fedora or CentOS** such as `No module named '_cffi_backend'`,
 install `python3-cffi` by typing:
 ```shell
     sudo yum install -y python3-cffi
 ```
+If your system does not provide the `python3-cffi` RPM package, you can install the `cffi` package with `pip`:
+```shell
+    sudo pip3 install cffi --target /usr/lib64/az/lib/python3.6/site-packages/
+```
 
 If you get errors on installation on **Debian or Ubuntu** such as the examples below,
 install libssl-dev and libffi-dev by typing:

doc/onboarding_guide.md

@@ -1,7 +1,9 @@
 Onboarding Best Practices
 =========================
 
-Reach out to `azpycli@microsoft.com` to get the process started. You'll be assigned a dev contact on the CLI team. Early and frequent communication with this contact is essential to ensuring a smooth onboarding.
+As a prerequisite, please contact Carl Shipley first for the [PLR](http://aka.ms/plrcriteria) (Product Launch Readiness) process.
+
+Then reach out to `azpycli@microsoft.com` to get the CLI onboarding process started. You'll be assigned a dev contact on the CLI team. Early and frequent communication with this contact is essential to ensuring a smooth onboarding.
 
 ## Extension vs. Module