docs: Update docs for using delta

[KapJI]

Delta documentation suggests to set it only as pager for Git, not as diff command. For chezmoi we need the same to have paging work correctly. Pager should be configured for delta separately, either as command line flag, .gitconfig or BAT_PAGER env var.

Closes #4205

[twpayne]

Thanks very much for this!

I'd like to do some more testing before merging this. chezmoi's integration with external diff tools and pagers is a bit of a mess for both inherent complexity and legacy reasons.

Inherent complexity:

• Some diff tools act like diff, taking filenames as arguments, and producing a diff, i.e. you use them as a diff tool.
• Other diff tools act as pagers, reading git diff-like input from stdin and reformatting the diff, i.e. you use them as a pager.
• Many diff tools add color to the output and/or expect to have control of the terminal.
• Many diff tools can only handle differences between existing regular files, and fail messily when presented with anything else, e.g. adding or removing a file, adding a directory, replacing a symlink with a device node, etc.
• Also, the interaction varies across operating systems (e.g. Windows requires special terminal handling).

Legacy reasons:

• chezmoi initially supported setting the pager, and this has since grown into pagers, diff commands, diff pagers, and all the associated complexity with managing these while maintaining backwards compatibility for existing users. Right now, it's a bit of a mess.