Fix #701: Move installation page to the top level of the docs and remove duplicate pages

[dashohoxha]

• Move installation page to the top level of the docs (currently it is under the user-guide).
• Remove https://dvc.org/doc/user-guide/plugins (which is now located under installation). See user-guide: reorganize installation page into this section #695 (review)
• Remove https://dvc.org/doc/user-guide/development (which is now located under installation). See details in user-guide: duplicate dev version docs (with different methods) #700.
• Simplify https://dvc.org/doc/get-started/install and maybe merge it with the next section (https://dvc.org/doc/get-started/initialize). See user-guide: reorganize installation page into this section #695 (review)
• Find all links to the removed pages and redirect them to the new pages.

Fix: #701, #660

[shcheklein]

Good stuff. I'm not sure if it's important enough to keep it at the very top level but we can change this in the future. Please, see a few comments and requests for further simplification.

[dashohoxha]

I'm not sure if it's important enough to keep it at the very top level but we can change this in the future.

Since we just link to it from get-started, I think that it should be the first item on the left menu, before Get Started. But it can be fixed later if needed.

[shcheklein]

@dashohoxha yep, agreed, we can def make it go before get started

[shcheklein]

@dashohoxha @jorgeorpinel had to revert it back since it affected the way dvc.org/doc (main page) looks like. Before it was using Get Started index as a default one and it was crafted to look nice and it makes sense to start with get started :) Install index page is not ready to be a front page for the docs website. Need more work here.

[jorgeorpinel]

Moving discussion to #747.