user-guide: merge/remove duplicate pages from new Installation subsection

[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)
• Update https://dvc.org/doc/user-guide/development. 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.

Depends on: #695
Related: #700 , #660

[jorgeorpinel]

Awesome. This would be worked on immediately after #695 is closed?

And can we include adding relevant Updating DVC notes here? (See #695 (review).) Or should I open a separate issue?

[shcheklein]

@jorgeorpinel I would keep it as a separate ticket. It's not a simple one :)

[jorgeorpinel]

OK, moved to #702

[jorgeorpinel]

Remove https://dvc.org/doc/user-guide/development (which is now located under installation). Same as #700?

Duplicates #700 but OK, we can consolidate the issues here 🙂