Config strings are messy AF

**Describe the bug**

In hugo.toml there's a load of random strings, like root, api, org, main_org_name because they've just grown up over time as we have discovered more and more string manipulation requirements

**What is wrong with this content?**

This is unfriendly lore for new users.

**How could this be fixed?**

Instead let's have a clearer taxonomy, something like

```
gitHubPrefix = 'https://github.com/"
gitHubAPI = "https://api.github.com/repos/"
gitHubOrg = "CodeYourFuture"
gitHubRootRepo = "curriculum"
gitHubPD = "CYF-PD" ??
```
And then go round and update all the calls in the regexes

**Link to content**

hugo.toml

**Additional context**

**Who else might need to know about this?**