Welcome to "Hullo World"
It's a simple self-hosted notebook web app modeled on Tiddlywiki classic version. Try it here. Live demo.
There is an index of links to cards at the bottom of the page. You can create a new card by clicking on the New button at the top of the page, or you can write a title in WikiTitle format and it will automatically be turned into a link that will create that card when you click on it.
You can save the current content by clicking on the Save... button at the top of the page. Your browser will prompt you for saving.
On a card clicking the Edit button will switch it to an edit mode that lets you change the title or edit the content. The content is markdown and the title should conform to WikiTitle. The Edit button changes to Done and the Close button (which closes the card) is disabled so you can't accidentally close the card during editing. (Yes, it might be preferable to auto-save on close, but whatever. This was easy and it works well enough for now.)