Docs: reordered index grid (#345)

* reordered index to have separate install card, and diataxis grid

* add Tutorials folder

* Explain that tutorials/README.md is just a blank placeholder file.  Can be removed in future.

* Update docs/index.rst

Co-authored-by: Leo Paoletti <164940351+lpaoletti@users.noreply.github.com>

* changed to quick start (no hyphen)

* missed index.rst for quick start edit

* change text to Using rocDecode

---------

Co-authored-by: Leo Paoletti <164940351+lpaoletti@users.noreply.github.com>

[ROCm/rocdecode commit: 7f5cc9c42f]

projects/rocdecode/docs/tutorials/README.md

@@ -0,0 +1 @@
+Placeholder to ensure docs/tutorials folder is created.  Can remove once tutorial content is provided.