text: Title: Better documentation about slack_sdk.models usage in Web APIs
  Body: We can improve the docstrings and the documentation website to provide more information about models.
  
  * Add more info about models support in chat.* API docstrings
  * Add a section about models in the website: https://slack.dev/python-slack-sdk/
  
  
  ---
  
  Interesting thanks! I definitely think this area of the package is pretty useful. IMO: it is a lot easier to read and understand from a code perspective than a large nested dictionary, and if someone is not using the block kit builder it is definitely a lot easier to write the code using the models.
  
  > No, you can directly pass an array of model objects as blocks keyword argument.
  
  Thanks, I would have had no idea about this, the doc string of `chat_postMessage` does not suggest this is possible.
  
  > For the above reason, we haven't been promoting the feature.
  
  `slack_sdk.models` is mentioned in the first few paragraphs of the readme ("easy-to-use builders"), maybe worth adding a note as this might mislead other developers as it has me.
  
  _Originally posted by @invokermain in https://github.com/slackapi/python-slack-sdk/issues/986#issuecomment-811106960_