Fix #403 by moving documentation to separate directory to avoid merge conflicts (#405)

docs/advanced/paths.md

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+---
+title: Paths in Quartz
+---
+
+Paths are pretty complex to reason about because, especially for a static site generator, they can come from so many places.
+
+A full file path to a piece of content? Also a path. What about a slug for a piece of content? Yet another path.
+
+It would be silly to type these all as `string` and call it a day as it's pretty common to accidentally mistake one type of path for another. Unfortunately, TypeScript does not have [nominal types](https://en.wikipedia.org/wiki/Nominal_type_system) for type aliases meaning even if you made custom types of a server-side slug or a client-slug slug, you can still accidentally assign one to another and TypeScript wouldn't catch it.
+
+Luckily, we can mimic nominal typing using [brands](https://www.typescriptlang.org/play#example/nominal-typing).
+
+```typescript
+// instead of
+type FullSlug = string
+
+// we do
+type FullSlug = string & { __brand: "full" }
+
+// that way, the following will fail typechecking
+const slug: FullSlug = "some random string"
+```
+
+While this prevents most typing mistakes _within_ our nominal typing system (e.g. mistaking a server slug for a client slug), it doesn't prevent us from _accidentally_ mistaking a string for a client slug when we forcibly cast it.
+
+Thus, we still need to be careful when casting from a string to one of these nominal types in the 'entrypoints', illustrated with hexagon shapes in the diagram below.
+
+The following diagram draws the relationships between all the path sources, nominal path types, and what functions in `quartz/path.ts` convert between them.
+
+```mermaid
+graph LR
+    Browser{{Browser}} --> Window{{Body}} & LinkElement{{Link Element}}
+    Window --"getFullSlug()"--> FullSlug[Full Slug]
+    LinkElement --".href"--> Relative[Relative URL]
+    FullSlug --"simplifySlug()" --> SimpleSlug[Simple Slug]
+    SimpleSlug --"pathToRoot()"--> Relative
+    SimpleSlug --"resolveRelative()" --> Relative
+    MD{{Markdown File}} --> FilePath{{File Path}} & Links[Markdown links]
+    Links --"transformLink()"--> Relative
+    FilePath --"slugifyFilePath()"--> FullSlug[Full Slug]
+    style FullSlug stroke-width:4px
+```
+
+Here are the main types of slugs with a rough description of each type of path:
+
+- `FilePath`: a real file path to a file on disk. Cannot be relative and must have a file extension.
+- `FullSlug`: cannot be relative and may not have leading or trailing slashes. It can have `index` as it's last segment. Use this wherever possible is it's the most 'general' interpretation of a slug.
+- `SimpleSlug`: cannot be relative and shouldn't have `/index` as an ending or a file extension. It _can_ however have a trailing slash to indicate a folder path.
+- `RelativeURL`: must start with `.` or `..` to indicate it's a relative URL. Shouldn't have `/index` as an ending or a file extension but can contain a trailing slash.
+
+To get a clearer picture of how these relate to each other, take a look at the path tests in `quartz/path.test.ts`.