add @blocknote/code-block installation guide on code-blocks page

[YousefED]

Discussed in #2884

^{Originally posted by lukonik July 4, 2026}
Hi there, in order to use default codeBlockOptions in code Block we need to install @blocknote/code-block package, but it is not explained in the Docs and My first thought was that this feature was deprecated. Doc Link

I propose to add installation step for better clarity :)