Joe bot docs: fix the broken link, add the list of commands

Why is this change being made?

Hotfixes for just merged !24156 (merged):

• fix: the design doc has a broken link to the blueprint doc.
• mention the blueprint in one more place in the design doc.
• provide the list of chatbot commands, basically describing the chatops interface – helpful for readers who are starting to use Joe.
• a couple of links to the design doc to make it visible from.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what