Add/document keyboard shortcuts for headlines in document editor

### Actual behavior
When writing a document, e.g. https://www.notebook.ai/documents/125118/, I can use `Ctrl+[B,K,I,U]` to make `[bold,link,italic,underlined]` text. The formatting bar does not show these key combinations.

### Expected behavior

I expected some key combinations to apply headlines. I tried the ones Google Docs uses [0] to no avail. I also tried Alt+[1,2,3] to format which did not work either. It is unclear to me if the keyboard shortcuts are missing or I just didn't try the correct ones.

Note: Ctrl+[1,2,3] should not be used as it is used for selecting tabs directly. Alt+[1,2,3] may have similar issues as Alt is often used as command key in programs.

### Steps to reproduce the behavior
Edit a document.

[0]

* Ctrl+] or [ (Windows/Chrome OS) and Cmd+] or [ (macOS): Increase or decrease paragraph indentation.
* Ctrl+Alt+0 (Windows/Chrome OS) and Cmd+Option+0 (macOS): Apply normal text style.
* Ctrl+Alt+1-6 (Windows/Chrome OS) and Cmd+Option 1-6 (macOS): Apply heading style [1-6]

https://www.howtogeek.com/439952/all-of-the-best-google-docs-keyboard-shortcuts/

Start in the document editor by reproducing the behavior while editing a document and comparing the formatting bar with the existing Ctrl+B, K, I, and U shortcuts. Determine whether headline shortcuts are missing or merely undocumented, then verify that the chosen shortcuts work without conflicting with tab selection and that the formatting bar documents them.

Written by the indexing model from the issue text.