Add an example with Buttons to self-assign roles

[SlimShadyIAm]

Summary

This is a simple example of how to create a persistent view with Buttons for self-assignable roles. Similar to previous approaches with reaction roles.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, ...)

[Lulalaby]

Cool thing. Gonna take a look later

[SlimShadyIAm]

I've simplified the example by removing emojis from the buttons and also added more documentation.