docs: Add lazy.nvim to installation docs

[woodevin]

No description provided.

[josealvaradoo]

Great 👌🏻

[Balake]

@woodevin I had to do lazy = false, not sure if that's true for all installations.

[amiv1]

@woodevin I had to do lazy = false, not sure if that's true for all installations.

It's better to specify either event or a command to make it load on demand, instead of during nvim startup.

{
  "sindrets/diffview.nvim",
  cmd = { "DiffviewOpen" },
}

Docs: https://lazy.folke.io/spec#spec-lazy-loading