[FEATURE] _Add SLANG to spec dropdown with a "landing page"

[marty-johnson59]

KhronosGroup/Vulkan-Guide#373 is Sascha's PR for the landing page content. Once that is accepted, #173 should be merged to link it into the top navbar.

(edit @oddhack: I removed the issue boilerplate and added links to the two open PRs which will address this.)

[marty-johnson59]

Add SLANG to spec dropdown with a "landing page" linking to relevant SLANG resources

[oddhack]

I created KhronosGroup/Vulkan-Guide#369 as the framework for a landing / resources page. That seems like the most sensible repository to put it in. Once the Guide page is merged, a small change to the site navigation in #173 will wire it into the top nav.

[SaschaWillems]

Since I did the HLSL page, I'd be willing to take this, unless someone else is already working on it.

[marty-johnson59]

Awesome!

[oddhack]

Updated link to Guide PR and navigation PR in issue description now.