Add Processing Instructions guide

### MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/ProcessingInstruction

### What specific section or headline is this issue about?

Description

### What information was incorrect, unhelpful, or incomplete?

We should consider moving/duplicating some of this data to a Processing Instructions guide rather than only having this under the `api` page.

This is especially relevant with Processing Instructions becoming more of a thing in HTML as part of Chrome's Declarative Partial Updates: https://developer.chrome.com/blog/declarative-partial-updates#out-of-order_streaming.

See also the discussion here: https://github.com/mdn/browser-compat-data/pull/30292#issuecomment-5409628333

### What did you expect to see?

A separate Processing Instructions guide.

### Do you have any supporting links, references, or citations?

_No response_

### Do you have anything more you want to share?

_No response_

Start with the ProcessingInstruction MDN reference page and the Chrome Declarative Partial Updates article, then compare the existing information under the API page. Define the scope and placement of a separate Processing Instructions guide, using the linked browser-compat-data discussion for context; done means the guide exists and the relevant reference content is appropriately moved or duplicated.

Written by the indexing model from the issue text.