Feature: bring all examples into the monorepo

[etrepum]

The homepage examples are all in the examples folder of the monorepo which makes it easy for anyone on the team to keep them maintained and up to date.

There are other examples in the documentation that point to codesandbox that are owned by individual developers which are harder to manage.

• https://codesandbox.io/s/lexical-plain-text-example-g932e
• https://codesandbox.io/s/lexical-rich-text-example-5tncvy
• https://codesandbox.io/embed/ecstatic-maxwell-kw5utu?fontsize=14&hidenavigation=1&module=/src/Editor.js,/src/plugins/CollapsiblePlugin.ts,/src/nodes/CollapsibleContainerNode.ts&theme=dark&view=split

I suggest we copy these to the examples folder and change the links to use StackBlitz as we are using on the homepage.

We could also update the PR template to include StackBlitz as an example instead of CodeSandbox.

CodeSandbox is a bit of a mess because they limit free accounts to 20 sandboxes, so it will make it much more likely that a sandbox on an older issue, discord discussion, or even the docs ends up getting deleted.

[etrepum]

It looks like the plain text example is 404 on codesandbox, so the problem has already presented itself 😆

[etrepum]

@acywatson is there any value in having the slightly more complex https://codesandbox.io/s/lexical-rich-text-example-5tncvy example vs. the one we have in-tree at https://stackblitz.com/github/facebook/lexical/tree/main/examples/react-rich (just noticed that the readme needs updating in that one! 😆) - and do you still have the source lying around for the plain text example? I found this one on github which seems like it was probably forked from the example before it disappeared https://github.com/lohocc/lexical-plain-text-example