Web extension concept page: making network requests

### MDN URL

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions

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

_No response_

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

Performing network requests is a very common need in browser extensions, but we don't have a page dedicated to describing the concepts.

MDN has documentation on fetch (https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch), so we only need to explain the relevance of extension-specific details: host permissions, the default CSP, and this mostly applies to cross-origin requests from extension contexts (and not content script contexts, which at least in MV3 do not have elevated cross-origin permissions).

Here is a dated example of Chrome's documentation (using XMLHttpRequest, which is the ancient predecessor to fetch): https://developer.chrome.com/docs/extensions/mv3/xhr/

Also, we bury some permission requirements under https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/host_permissions (and for MV2, https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions). These pages, along with the new CSP section, should be linked from a concept page that explains how to perform a network request.

### What did you expect to see?

New making network requests concepts page

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

See #23114

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

_No response_