Add release notes pages #19

There is a page that shows the release notes for every major MTA version. Every major version has had release notes published on the wiki (e.g. last release https://wiki.multitheftauto.com/wiki/Changes_in_1.6 and current release WIP https://wiki.multitheftauto.com/wiki/Changes_in_1.7).

This is not the same as the News post or Release on GitHub that contain more user friendly text and other content intended for the public.

I believe that the release notes could be generated by structured files containing the various changes labeled properly, like deprecations, notable changes, new features, updates, etc. All properly categorized. It should also contain additional sections for notes like Important information etc.

Example files:

• "release_notes/1.7.0/new_features.md"
• "release_notes/1.7.0/vendor_updates.md"

We could use markdown files enriched with custom makros/syntax to use constants and variables and more easily format things.

In my opinion it's better to use different files because the changelogs are huge.