Move GNOME Shell Extensions Review Guidelines to gjs.guide

Why

Currently we are using extensions rebooted repo to manage the changes in extensions review guidelines. After the approval, we are updating the review guidelines wiki page manually to apply the changes. This approach has some cons:

• We have to apply changes in html while we had the changes in markdown.
• We have to apply changes manually after approval.
• We may forget to apply changes to the wiki page or extensions rebooted repo (in case it was just an issue not MR).

Moving the review guidelines to gjs.guide can have some benefits:

• We don't need extensions rebooted repo to fix issues. All change requests can happen in gjs.guide repo.
• We don't need to convert markdown to html. This happens automatically here.
• Contribution on GitLab is much easier than wiki.gnome.org.

This MR

This MR is a markdown version of the review guidelines:

• Has been converted from the recent wiki page manually.
• Removed dead link (https://wiki.gnome.org/TypeScript).
• Updated GNOME HIG link (https://developer.gnome.org/hig/stable/ to https://developer.gnome.org/hig/).
• Removed schema xml file from unnecessary files section (We removed it previously but somehow it got overridden in the latest update).