Add 'Command' explanation to .aseprite-keys documentation

[Arecher]

Added a paragraph to explain that you need to create custom commands to be able to hook up custom key shortcuts to extension functionality.

Added link to API documentation for :NewCommand(). Since it links from the Docs repository to the API repository, I had to use a hardcoded website link. If there is a better way to do that, let me know

[Arecher]

I added this explanation because when I first encountered .aseprite-keys I couldn't for the longest time figure out how to set them up. Even though it clearly states 'Command', I didn't realise it wanted a 'NewCommand()' name.

I tried function names, plugin names, command ids etc. Took me a bit, so I thought I would help out anyone else that might have read the page a little too quickly!

[dacap]

Thanks for this @Arecher!

[Arecher]

When checking my local branches earlier today, it told me I needed to sync to the latest version of the main repository. I did that because I wanted to be on the latest version, so I could safely make another commit (which was actually part of the API repository instead). And as far as I understood, the old commits were already safe and sound in this push request anyway, but it seems that the change caused this request to be closed and the file changes removed?

I'm not very familiar with Github, so perhaps I synced the branch too early, or simply synced it wrong. If this means my documentation changes can no longer be added to the main repository, let me know and I'll make another commit (in here, if I can figure that out!)

[dacap]

Probably the problem was that you’ve creates this PR from your own “main” branch but then synced it with our “main”, so your branch was hard reset.

You should create a new branch to create the PR, so when you sync main your PR is kept open (until your branch is deleted or merged).

[Arecher]

Thanks for the explanation, that makes a lot of sense! It seems you cannot change which branch you commit from after making a pull-request, so the recommended action was to make a completely new one. Sorry for the inconvenience and confusion!