Document the different diagnostic levels

[jerabaul29]

In https://github.com/helix-editor/helix/blob/master/book/src/editor.md , I could not find a documentation for the valid diagnostic levels; for example, in section

https://github.com/helix-editor/helix/blob/af7a1fd20c0a2915e0dae1b5bea7cb6bde6c2746/book/src/editor.md?plain=1#L399-L435

it is not clear to me, reading just this section, what values are ok to use for e.g. cursor-line .

[jerabaul29]

Ok, after a bit of digging I could find this information at:

https://github.com/helix-editor/helix/blob/af7a1fd20c0a2915e0dae1b5bea7cb6bde6c2746/book/src/languages.md?plain=1#L63

not sure how this could be made easier to find? It was a bit of digging to find it. Should we add "see diagnostic-severity for admissible values", or "(Allowed values: Error, Warning, Info, Hint)", or something like this? Not sure how to document this effectively without putting the same boilerplate text again and again.

[the-mikedavis]

I think we can add a paragraph below the table in this section that says that the allowed values are "hint", "info", "warning" and "error". I think it would be ok to reference some other config option for config.toml but linking to the languages.toml config could be confusing. Also that section in languages.md is incorrect about the levels: it should be lowercase "hint"/"info"/etc. instead of "Hint"/"Info"/etc.