ENH: Add inline reference button

For https://gitlab.indiscale.com/caosdb/customers/cfaed/management/-/issues/29. Introduces another button that includes an inline-reference template

`[linkanme](entity:id.prop)`

at the current position. Also adds some development documentation to the README.md.

For testing, you can use npm install and npm run start in this repo. To actually test it in CaosDB's WebUI, either npm pack this repository and include it in our fork of the ckeditor5 or use the f-inline-reference-button branch in that repo. Then, build and include the CKEditor as described in the README_INDISCALE.md and test it in CaosDB's WebUI.

sample/ckeditor.js

@@ -104,7 +104,8 @@ ClassicEditor
 			'codeBlock',
 			'|',
 			'inlineMathButton',
-			'displayMathButton'
+			'displayMathButton',
+			'inlineReferenceButton'
 		],
 		image: {
 			toolbar: [ 'imageStyle:inline', 'imageStyle:block', 'imageStyle:side', '|', 'imageTextAlternative' ]