add "How RetroAchievements Works" page

[suspect15]

Adds page that gives a general explanation of how RetroAchievements works. The purpose is to both explain achievement awarding functionality to curious players and hopefully inspire potential future developers to feel capable of learning how to produce achievement sets.

[wescopeland]

Recommend adding a frontmatter title and description to the new page. Examples can be found on any of the flags pages in the dev docs. This is the format:

---
title: This is my title
description: This is my description that is ideally 220 chars or less
---

This is mostly also to help search engines and Discord embeds. The title & description are what search engines and embeds will display.

[wescopeland]

Thanks for working through the feedback - looks good to me