Enable rockspec

[sankantsu]

Closes #191

This PR introduce following changes:

• Rename *.rockspec.disabled -> *.rockspec to enable lazy.nvim can detect rockspec.
• Simply use default branch URL for src.url (no need to manual update).
• Update install instruction for lazy.nvim >= v11.

Re-enabling .rockspec file benefits lazy.nvim users, enabling automic build of image.nvim and magick rocks. It eliminates the need to manually change package.path.

References:

• Lazy.nvim
  • Lazy.nvim: https://lazy.folke.io/packages
  • Lazy.nvim issue: luarocks needs lua 5.1 folke/lazy.nvim#1570
• Luarocks
  • Creating a rock: https://github.com/luarocks/luarocks/wiki/Creating-a-rock
  • Rockspec format: https://github.com/luarocks/luarocks/wiki/Rockspec-format

Disclaimer:
I'm not an expert of Lua and Luarocks.

[sankantsu]

I noticed that @kevinm6 also suggested similar documentation changes in #193.
Both (this PR and #193) proposes opts.rocks.hererocks = true. But it seems that rocks configuration within defaults does not work for me (hererocks will be not installed and :checkhealth still complains).
c.f. https://lazy.folke.io/configuration

By the way, you can pick some changes from #193, if you think it's appropriate.

[3rd]

Thank you, let's merge this one first.
@kevinm6 do you think we need to add documentation from #193 ?

[kevinm6]

I don’t think so @3rd, I’ll close that PR !
Maybe you could move #191 as a Discussion since it can be useful to some people.