[Feature]: Display description to a topic on the corresponding topic page

[alx-net]

As a user, I want to read an explanation of the topic after I open it.

E.g. When the user click "data type", there is a short description for it.

Acceptance Criteria:

• [x] On a topic subpage there should be a description of the topic, so the user is able to understand it
• [x] The description should include examples