User docs for web UI and API

This MR adds user docs for web UI and web API guides to the docusaurus setup of the unhide docs

Closes #13 (closed), #12 (closed)

docs/guides/web-api/categories-api.md

+---
+displayed_sidebar: guidesSidebar
+sidebar_position: 3
+---
+
+# /categories
+
+This API provides the user information on the basic categories by the which the information in the HKG is grouped, and the total number of records found for each of this categories, respectively.
+
+### Method
+
+GET
+
+### Search Params
+
+None
+
+### URL
+
+[https://search.unhide.helmholtz-metadaten.de/api/categories](https://search.unhide.helmholtz-metadaten.de/api/categories)
+
+### Response Model
+
+```
+[
+  {
+    id: string,
+    text: string,
+    count: number
+  }
+]
+```
+
+where, `id` is the identifier of the category, `text` is the name of the category to be displayed on the UI, and `count` presents the total number of records available for the given category
+
+#### Example
+
+```
+[
+      {
+        id: "datasets",
+        text: "Datasets",
+        count: 411935,
+      },
+      {
+        id: "software",
+        text: "Software",
+        count: 3521,
+      },
+      {
+        id: "documents",
+        text: "Documents",
+        count: 1679342,
+      },
+      {
+        id: "instruments",
+        text: "Instruments",
+        count: 0,
+      },
+      {
+        id: "experts",
+        text: "Experts",
+        count: 229601,
+      },
+      {
+        id: "institutions",
+        text: "Institutions",
+        count: 64472,
+      },
+      {
+        id: "trainings",
+        text: "Trainings",
+        count: 667,
+      },
+      {
+        id: "projects",
+        text: "Projects",
+        count: 0,
+      },
+    ]
+```