Write clear `lead` variables for the `/concepts` section.

[johnnymatthews]

The lead variable controls show is shown directly under the title of a page.

The purpose of this text is to quickly tell the user what they can find in this page. However, for the /concepts section, the lead should quickly define the title of the page. For example:

---
title: "Programs"
lead: "The purpose of an Entropy program is to determine whether a group of nodes should generate a signature or not. Developers can create and deploy programs, but validator nodes are the only agents that will directly interact with the programs once deployed. Programs do not return any data other than a success or failed response."
---

[...]

Tasks

• Create appropriate lead text for each page within the /concepts section:

[johnnymatthews]

This was updated recently in #238. Closing.