feat: add ionic's wanted/basic Content-Security-Policy config into the docs (or src/index.html?)

# Feature Request

**Ionic version:**
[x] **5.x**

**Describe the Feature Request**
Add information about how to support new protocols on `Content-Security-Policy ` without break Ionic.

**Additional Context**
I want to support the cdvfile protocol and it requires to add cdvfile: to the `Content-Security-Policy` meta tag. Since no one exist I have to play adding and removing tags until I get no more errors in the console.

I was not able to find what was the current "default" `Content-Security-Policy` by an Ionic 5 project.

This is also needed to support web sockets.

Start by inspecting src/index.html and the Ionic 5 documentation for the current Content-Security-Policy setup. Document the default policy and explain how to add cdvfile and WebSocket protocols without breaking it, with the final location resolved between the docs and src/index.html.

Written by the indexing model from the issue text.