Rename def-env to def --env

[arnau]

This PR changes all def-env to def --env. def-env was deprecated in 0.87.

There are a few commits:

• Rename def-env to the new def --env: Changes the book in English
• (de) Rename def-env to the new def --env: Changes the book in German (I don't speak German)
• (zh-CN) Rename def-env to the new def --env: Changes the book in Chinese (I don't speak Chinese)
• Update command docs. Change after running nu make_docs.nu
• Add docs for the experimental command category. Please review this bit. I was not expecting nu make_docs.nu to generate these files. It seems like it's due to a plugin I have installed which makes me question whether make_docs.nu should filter plugins out?.

╭────────────────────┬───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ version            │ 0.87.1                                                                                                                │
│ branch             │                                                                                                                       │
│ commit_hash        │ 3d631490bc17cfef2944072f9b7869cd9cac1e7b                                                                              │
│ build_os           │ macos-aarch64                                                                                                         │
│ build_target       │ aarch64-apple-darwin                                                                                                  │
│ rust_version       │ rustc 1.71.1 (eb26296b5 2023-08-03)                                                                                   │
│ rust_channel       │ 1.71.1-aarch64-apple-darwin                                                                                           │
│ cargo_version      │ cargo 1.71.1 (7f1d04c00 2023-07-29)                                                                                   │
│ build_time         │ 2023-11-25 17:11:03 +00:00                                                                                            │
│ build_rust_channel │ release                                                                                                               │
│ allocator          │ mimalloc                                                                                                              │
│ features           │ dataframe, default, extra, sqlite, trash, which, zip                                                                  │
│ installed_plugins  │ custom-value generate, custom-value generate2, custom-value update, from eml, from ics, from ini, from vcf, gstat,    │
│                    │ inc, query, query json, query web, query xml                                                                          │
╰────────────────────┴───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯```

[amtoine]

@arnau
could you revert the changes to the commands/ directory so that we keep this PR focused on the def-env fix? 😇

let's say the diff is... huge 😉

[arnau]

reverted 👍

[amtoine]

thanks for going through these, just a few remarks 😉

[arnau]

@amtoine could you have a fresh look please? I've done it now from scratch without git hooks. All markdown issues are gone as far as I can tell.

[amtoine]

oooh yeah, looks much better, thanks a lot 🙏

hopefully the last def-env are gone for good 😌