docs: add docs for generating docs for nixlet values, update nixmkdocs

docs/generating_docs.md

@@ -0,0 +1,38 @@
+# Generating Docs
+
+Like you can see on the left (if you are looking at the built MkDocs site), it's possible
+to generate docs for Nixlets' values automatically.
+Since the values are basically just Nix module options, we can generate docs similarly to NixOS options etc.
+
+## Generate Markdown
+
+This is all that's needed:
+
+```nix
+(<nixlet>).mkDocs {
+# Params:
+#  transformOptions ? opt: opt,
+#  filter ? _: true,
+#  headingDepth ? 3,
+}
+```
+
+This will return the path to a markdown file containing the docs, like this:
+
+````md
+### `example`
+
+(no description)
+
+**Type**:
+
+```console
+string
+```
+
+**Default value**:
+
+```nix
+"Hello world!"
+```
+````