Various improvements to the atuin import page

[waldyrious]

This PR implements a few improvements to the import doc page.

1. Fix name of zsh-hist-db and xonsh-sqlite subcommands

   The atuin import subcommands zsh-hist-db and xonsh-sqlite
   were incorrectly documented as zsh_histdb and xonsh_sqlite, respectively.

   This is likely due to the actual modules that implement the import functionality
   being indeed named zsh_histdb and xonsh_sqlite (see here and here),
   but for the end-user, that results in documentation that's confusing at best,
   and error-inducing at worst.

   Fixes issue atuinsh/atuin#2533.

2. Document the powershell and replxx import subcommands

   These importers were introduced in PRs atuinsh/atuin#2864 and atuinsh/atuin#2024, respectively,
   but the documentation was not updated to account for them.

3. Fix alphabetic sorting of sections

   Move the nu-hist-db and zsh-hist-db sections to be after nu and zsh, respectively.
   This is more intuitive, and matches the existing sort used for the xonsh / xonsh-sqlite sections.

4. Minor whitespace corrections

   Fix line wrapping in the resh section,
   and remove trailing whitespace in the nu* sections.

[waldyrious]

Note/suggestion for maintainers: items 1 and 3 perhaps indicate that the docs contents should probably live inside Atuin's main repository, to reduce the likelihood of the docs getting out of sync with the code.