refactor(docs): dev-guide: write initial styleguide page

docs/developers-guide/styleguide.md

+# Styleguide
+
+If not explicitly stated otherwise,
+the following takes effect for all files:
+
+- character encoding __utf-8__
+- line endings with __lf__
+- indentation with __4 spaces__
+- final new line
+
+In general,
+it's a good idea to have a look into the yet existing code,
+to check how certain things were done up to now.
+
+## Java
+
+The most important thing to say here is: opening braces are written into the next line.  
+_(more to come if s.o. does s.th. differently I didn't think of)_
+
+## Markdown
+
+!!! info "Todo"
+    write docs
+
+most important bullet points:
+
+- leave always an empty line above and beneath of lists.
+    It may render fine in normal markdown viewers,
+    but not in the MkDocs Pages docs (like this)  
+
+    ??? example inline end
+        __bad:__
+        ```markdown
+        some text
+        - bullet point
+        - another item
+        more normal text
+        ```
+        __good:__
+        ```markdown
+        some text
+
+        - bullet point
+        - another item
+
+        more normal text
+        ```
+
+- if you insert a link,
+    __always__ write the link to the bottom of the file with an alias,
+    ordered alphabetically.  
+
+    ??? example
+        __bad:__
+        ```markdown
+        Look [here](https://www.coolsite.not/with/ultra/long/url) to read more about this.
+        ```
+        __good:__
+        ```markdown
+        Look [here][coolsite] to read more about this.
+
+        [coolsite]: https://www.coolsite.not/with/ultra/long/url
+        ```
+
+- split long sentences on punctuation and connection words like `and` & `or`.  
+    It will have the same appearance in viewers,
+    but be much easier to maintain on code side.
+
+    ??? example
+        __bad__:
+        ```markdown
+        Some sentence that is very long, therefore it has punctuation and has absolutely no sense at all.
+        ```
+        > Some sentence that is very long, therefore it has punctuation and has absolutely no sense at all.
+
+        __good:__
+        ```markdown
+        Some sentence that is very long,
+        therefore it has punctuation
+        and has absolutely no sense at all.
+        ```
+
+        > Some sentence that is very long,
+        > therefore it has punctuation
+        > and has absolutely no sense at all.

mkdocs.yml

@@ -30,6 +30,7 @@ nav:
           - Ticketsystem: 'api/systems/gitlab/ticket-system.md'
   - Developers Guide:
     - Index: 'developers-guide/index.md'
+    - Styleguide: 'developers-guide/styleguide.md'
 
 theme:
   name: 'material'