Add documentation for Xonsh import (#29)

Documentation for Xonsh importers, as discussed in atuinsh/atuin#1678
and atuinsh/atuin#1711.

I also noticed that the TOC wasn't generating properly on the page, so I
changed the h1's to h2's and that seems to have fixed it. This does
slightly change the appearance of the page, so I'm happy to revert that
if you'd prefer to keep it the way it was.

src/content/docs/reference/import.mdx

@@ -2,7 +2,7 @@
 title: import
 ---
 
-# `atuin import`
+## `atuin import`
 
 Atuin can import your history from your "old" history file
 
@@ -19,19 +19,22 @@ control which file is read, otherwise each importer will try some default filena
 HISTFILE=/path/to/history/file atuin import zsh
 ```
 
+Note that for shells such as Xonsh that store history in many files rather than a
+single file, `$HISTFILE` should be set to the directory in whic those files reside.
+
 For formats that don't store timestamps, timestamps will be generated starting at
 the current time plus 1ms for each additional command in the history.
 
 Most importers will discard commands found that have invalid UTF-8.
 
-# bash
+## bash
 
 This will read the history from `$HISTFILE` or `$HOME/.bash_history`.
 
 Warnings will be issued if timestamps are found to be out of order, which could also
 happen when a history file starts without timestamps but later entries include them.
 
-# fish
+## fish
 
 fish supports multiple history sessions, so the importer will default to the `fish`
 session unless the `fish_history` environment variable is set. The file to be read
@@ -40,28 +43,53 @@ will be `{session}_history` in `$XDG_DATA_HOME/fish/` (or `$HOME/.local/share/fi
 Not all of the data in the fish history is preserved, some data about filenames used
 for each command are not used by Atuin, so it is discarded.
 
-# nu-hist-db
+## nu-hist-db
 
 This importer reads from Nushell's SQLite history database, which is stored in 
 `$XDG_CONFIG_HOME/nushell/history.sqlite3` or `$HOME/.config/nushell/history.sqlite3`.
 There is no way to set the filename otherwise.
 
-# nu
+## nu
 
 This importer reads from Nushell's text history format, which is stored in 
 `$XDG_CONFIG_HOME/nushell/history.txt` or `$HOME/.config/nushell/history.txt`.
 There is no way to set the filename otherwise.
 
-# resh
+## resh
 
 The [RESH](https://github.com/curusarn/resh) importer will read from `$HISTFILE` or `$HOME/.resh_history.json`.
 
-# zsh_histdb
+## xonsh
+
+The Xonsh importer will read from all JSON files it finds in the Xonsh history
+directory. The location of the directory is determined as follows:
+* If `$HISTFILE` is set, its value is used as the history directory.
+* If `$XONSH_DATA_DIR` is set (as it typically will be if the importer is invoked
+from within Xonsh), `$XONSH_DATA_DIR/history_json` is used.
+* If `$XDG_DATA_HOME` is set, `$XDG_DATA_HOME/xonsh/history_json` is used.
+* Otherwise, `$HOME/.local/share/xonsh/history_json` is used.
+
+Not all data present in Xonsh history JSON files is used by Atuin. Xonsh stores the
+environment variables present when each session was initiated, but this data is
+discarded by Atuin. Xonsh optionally stores the output of each command; if present
+this data is also ignored by Atuin.
+
+## xonsh_sqlite
+
+The Xonsh SQLite importer will read from the Xonsh SQLite history file. The history
+file's location is determined by the same process as the regular Xonsh importer,
+but with `history_json` replaced by `xonsh-history.sqlite`.
+
+The Xonsh SQLite backend does not store environment variables, but like the JSON
+backend it can optionally store the output of each command. As with the JSON backend,
+if present this data will be ignored by Atuin.
+
+## zsh_histdb
 
 This will read the Zsh histdb SQLite file from `$HISTDB_FILE` or
 `$HOME/.histdb/zsh-history.db`.
 
-# zsh
+## zsh
 
 This will read the Zsh history from `$HISTFILE` or `$HOME/.zhistory`
 or `$HOME/.zsh_history` in either the simple or extended format.