Add guidance for keyboard shortcuts for international keyboards

[JasonWeill]

Related indirectly to #7579.

Document best practices for selecting a keyboard shortcut for a new command. In particular, favor using letters A–Z and digits 0–9 and avoid using symbols such as backtick, tilde, and brackets.

Problem

There are hundreds of keyboard layouts that are in use in the world, representing a combination of countries, languages, operating systems, and user preferences (QWERTY vs. Dvorak, for example). Many European keyboards have both a SHIFT key and an ALT GR (distinct from ALT) key which are used to produce an alternate symbol. Common programming symbols such as { and } require a US English keyboard user to press SHIFT and a key, but a Norwegian user would have to press ALT GR and a key. The backtick key may be present only as a "grave accent" dead key, which requires a user to press a second key to display anything (e.g., grave accent, E produces è).

Proposed Solution

Amend the documentation to recommend that when users add a new keyboard shortcut for their command, they use only letters A–Z or digits 0–9, avoiding symbols, as symbols may not be available on all international keyboards.

Additional context

Many keyboard layouts are available on-line, such as the Norwegian keyboard layout mentioned above.

[isabela-pf]

There's a draft PR at the Lumino level that may start to address this issue (discussed in the JupyterLab weekly meeting).

[jasongrout]

@jweill-aws - this is an example of an issue that I think we can just take the needs triage label off. Do you agree?