docs: add API documentation page with code examples for user registration, encryption, and view creation

Add three new files: docs.html template extending base layout with back button and markdown frame, docs.md containing API usage examples for user registration, string encryption, and view classes, and docs.py with DocsHTMLView and DocsMDView handlers rendering the respective templates.

src/snek/templates/docs.html

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+{% extends "base.html" %}
+
+{% block main %}
+<div class="dialog">
+
+    <fancy-button size="auto"  text="Back" url="/back"></fancy-button>
+<html-frame url="/docs.md"></html-frame>
+
+</div>
+{% endblock %}

src/snek/templates/docs.md

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+# API Documentation
+
+Currently only some details about the internal API are available.
+
+## How to create a user
+```python
+# Save user to the table named 'user'
+# Password gets sha256 encrypted with default a salt string
+# of the snek.system.security module.
+
+new_user_object = await app.service.user.register(
+    username="retoor", 
+    password="retoorded"
+)
+```
+
+## Encrypt string
+```python
+from snek.system import security
+
+# Support for both utf and bytes.
+var1 = security.encrypt("data")
+var2 = security.encrypt(b"data")
+
+# Is correct:
+assert(var1 == var2)
+```
+
+## How to create a basic HTML / Markdown view
+```python
+from snek.system.view import BaseView 
+
+class IndexView(BaseView):
+    
+    async def get(self):
+        # The render function supports markdown.
+        # It will render with syntax highlighting.
+        # Just use the .md file extension in the file name.
+        return await self.render("index.html")
+```
+## How to create a FormView
+```python
+from snek.system.view import BaseFormView
+from snek.form.register import RegisterForm
+
+class RegisterFormView(BaseFormView):
+    
+    form = RegisterForm
+```
+## How to register a class view
+```python
+app.routes.add_view("/your-page.html", YourViewClass)
+```

src/snek/view/docs.py

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+
+
+
+from snek.system.view import BaseView
+
+
+class DocsHTMLView(BaseView):
+
+    async def get(self):
+        return await self.render_template("docs.html")
+    
+class DocsMDView(BaseView):
+
+    async def get(self):
+        return await self.render_template("docs.md")