docs: Add vim-plug installation instructions.

[southp]

The PR adds a vim-plug section in installation.md to show how to install blink.cmp via vim-plug.

[soifou]

Thanks for the PR! Here are some insights:

• This PR appears to resolve Improvement: add installation instructions for vim-plug #2216, you might want to take a look at that issue for inspiration. Then reference and close it in your commit message body.
• It would be helpful to clarify that the blink.cmp configuration shown here uses the default settings. You could mention this explicitly or comment them out in the example to make that clear.
• The sentence "config fetching the pre-built binary as the post-installation action" is a bit unclear. Consider rephrasing it to something like: "using a release tag to fetch the pre-built binary after installation" (similar to the lazy.nvim section).