Add development guide

[thinkall]

Why are these changes needed?

Looks like we don't have a development guidance in the documentation website for v0.4.

In this PR, a development guide is added to the website. Along with it, the poe commands are updated. Now poe check will run all the checks including building and serving docs in your local environment.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.