Reorganize and update installation instructions

[laniakea64]

• Update lazy.nvim installation instructions for current lazy.nvim. Our ftdetect changed since then and the existing instructions were no longer working consistently for me.
• Remove Pathogen installation instructions, Pathogen is officially dead for more than a year now.
• Promote Vim8 Package and use a dedicated subsection for plugin manager based installations:
  • Since Vim8 Package is built into Vim/Neovim, it's in a different category. And in the absence of a plugin manager, Vim8 Package would be the simplest installation method.
  • The previous organization looked like we specifically support all listed plugin managers. If we add more plugin managers instructions in future, we may want to add "Community-contributed installation instructions" in some form, to provide the helpful information for users without committing to supporting too many additional plugin managers. That would fit better in this organization.
• Add Repology badge to list third-party packages. Apparently Repology knows about this project now thanks to vimPlugins.vim-just: init at 2023-04-21 NixOS/nixpkgs#235765