docs: improve lazy install snippet

[joshmedeski]

The lazy.nvim install snippets can be improved:

1. opts = {} is all you need to setup the plugin, less code
2. It's recommended to add cmd = "GitLink" for lazy loading, the plugin will only load when GitLink is executed
3. keys offers an example (with ruifm's default keymapping) as an example. It will also lazy load the plugin if this keymap is triggered.

Regression Test

Platforms

• windows
• macOS
• linux

Hosts

• Test on github.com.
• Test on gitlab.com.
• Test on bitbucket.org.
• Test on codeberg.org.
• Test on git.samba.org.

Functions

• Use GitLink(!) to copy git link (or open in browser).
• Use GitLink(!) blame to copy the /blame link (or open in browser).
• Use GitLink(!) default_branch to open the /main//master link in browser (or open in browser).
• Use GitLink(!) current_branch to open the current branch link in browser (or open in browser).
• Copy git link in a symlink directory of git repo.
• Copy git link in an un-pushed git branch, and receive an expected error.
• Copy git link in a pushed git branch but edited file, and receive a warning says the git link could be wrong.

[linrongbin16]

thanks!