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 2 above 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.

[waldyrious]

@ellie apologies for the ping, but just making sure this doesn't get stale by lack of action on my part 😅

By the way, the fact that the main repository is considerably more active than this one (the two PRs I submitted to it were merged pretty quickly, despite having been created around the same time as this one) is yet another point reinforcing my suggestion in the previous comment, of moving the docs to the main repo :) let me know if that's something y'all would consider!