docs: Various improvements to the atuin import docs

[waldyrious]

Recreation of PR atuinsh/docs#111, after the migration of the docs to this repo (#3018).

Copying the description from that PR, for convenience:

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 correction

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

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

Fixes #2533.

[greptile-apps]

Greptile Summary

Documentation improvements for atuin import command reference. Corrects the names of the zsh-hist-db and xonsh-sqlite subcommands (previously incorrectly documented with underscores), adds missing documentation for powershell and replxx importers, improves alphabetic ordering by moving nu-hist-db and zsh-hist-db sections after their base importer sections, and fixes minor whitespace in the resh section.

• Fixed zsh_histdb → zsh-hist-db (correct kebab-case CLI name)
• Fixed xonsh_sqlite → xonsh-sqlite (correct kebab-case CLI name)
• Added powershell importer documentation with platform-specific paths
• Added replxx importer documentation
• Improved section ordering for better readability

Important Files Changed

Filename	Overview
docs/docs/reference/import.md	Documentation fixes: corrected subcommand names, added missing importers (powershell, replxx), improved alphabetic ordering