Update, expand, proofread documentation for vi

[Yilin-Yang]

Update outdated information in vi command reference, add more recommendations, perform general cleanup.

• Add more "Things to Learn" and more plugin recommendations.
• Reorganize plugin recommendations.
• Add a table of contents.
• Add a section on Language Server Protocol, and configuring LSP to work with vim.
• CORRECTLY IMPLEMENTED adaptive image resizing that doesn't make the CreativeCommons logo look weird! :D
• Fix typos and dead links.
• Miscellaneous style tweaks in the vi markdown file.
• Add external links.