feat: add demo devcontainer

[aaronsteers]

Partially resolves #196 (No demo workflow yet, but UI launches successfully.)

Important:

1. I've tested this locally in VS Code successfully - the postCreateCommand performs both the install of langflow from pip (via pipx) and also the launching of langflow UI. VS Code also helpfully told me when the new port had been opened and I could simply click the button to open the site in a browser tap.
2. Page functions in the UI seem to be working great.
3. I have not yet tested this in Codespaces, but something other than the default 2-CPU image might be warranted here.
4. The new README could use some more work, admittedly, but as of now at least, it tells the audience what to expect from the container.

Can be tested with this link: Create new codespace

You have to manually select the branch and also choose second option in the list of Devcontainer options:

[ogabrielluiz]

This looks great. I'll test it asap.

We are adding loading of Example from the langflow_examples repo so the demo should be pretty easy to run.