refactor: Add 'src' folder in 'site' (#445)

This refactoring re-organizes the `site` folder to have a nested `src` folder.

Originally, [we wanted to keep the directory structure shallow](https://github.com/coder/coder/pull/8#issuecomment-1009578910) - but there were two points that motivated this change to introduce the `src` level.

1. We have several non-`src` folders now (`e2e`, `static`, `html_templates`, `.storybook`)
2. Having a `src` folder makes it easier to run XState Typegen

So given those two data points - I believe it makes sense to revisit that and introduce a `src` folder.

site/.storybook/main.js

@@ -1,7 +1,7 @@
 const path = require("path")
 
 module.exports = {
-  stories: ["../**/*.stories.mdx", "../**/*.stories.@(js|jsx|ts|tsx)"],
+  stories: ["../src/**/*.stories.mdx", "../src/**/*.stories.@(js|jsx|ts|tsx)"],
   addons: ["@storybook/addon-links", "@storybook/addon-essentials"],
   babel: async (options) => ({
     ...options,

site/.storybook/preview.js

@@ -1,6 +1,6 @@
 import ThemeProvider from "@material-ui/styles/ThemeProvider"
 import { withThemes } from "@react-theming/storybook-addon"
-import { light, dark } from "../theme"
+import { light, dark } from "../src/theme"
 import { addDecorator } from "node_modules/@storybook/react"
 import { createMemoryHistory } from "history"
 import { unstable_HistoryRouter as HistoryRouter } from "react-router-dom"