docs: Add PlugY save data format info

docs/.gitignore

@@ -3,3 +3,4 @@ node_modules/
 html/
 xml/
 latex/
+package-lock.json

docs/SUMMARY.md

@@ -2,3 +2,7 @@
 
 * [Introduction](INDEX.md)
 * [API Reference](api.md)
+
+### File Structures
+
+* [PlugY Save Files (.sss/.d2x)](formats/plugy.md)

docs/book.json

@@ -1,5 +1,5 @@
 {
-	"plugins": ["theme-official","-sharing","-fontsettings","-search"],
+	"plugins": ["theme-official","-sharing","-fontsettings","-search","-lunr"],
 	"title": "d2itemreader",
 	"github_url": "https://github.com/squeek502/d2itemreader",
 	"structure": {

docs/formats/plugy.md

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+## PlugY Shared Stash (.sss) File Format
+
+Byte Offset | Size (bytes) | Description
+------------|:------------:|-------------
+0x00 | 4 | File header. Must be "SSS\0" (0x00535353 when read as a 32 bit unsigned integer).
+0x04 | 2 | File version. Will be "01" (0x3130) when file version is 1, or "02" (0x3230) when file version is 2.
+
+#### File version 1
+
+When the file version is "01", there will be no shared gold data in the file.
+
+Byte Offset | Size (bytes) | Description
+------------|:------------:|-------------
+0x06 | 4 | Number of pages in the stash data as a 32 bit unsigned integer
+0x0A | ... | Beginning of [Stash data](#stash-data-format)
+
+#### File version 2
+
+When the file version is "02", there will be shared gold data in the file.
+
+Byte Offset | Size (bytes) | Description
+------------|:------------:|-------------
+0x06 | 4 | Shared gold as a 32 bit unsigned integer
+0x0A | 4 | Number of pages in the stash data as a 32 bit unsigned integer
+0x0E | ... | Beginning of [Stash data](#stash-data-format)
+
+## PlugY Personal Stash (.d2x) File Format
+
+Byte Offset | Size (bytes) | Description
+------------|:------------:|-------------
+0x00 | 4 | File header. Must be "CSTM" (0x4D545343 when read as a 32 bit unsigned integer).
+0x04 | 2 | File version. Will be "01" (0x3130) when file version is 1. No other versions are in use currently.
+0x06 | 4 | Unused bytes
+0x0A | 4 | Number of pages in the stash data as a 32 bit unsigned integer
+0x0E | ... | Beginning of [Stash data](#stash-data-format)
+
+## Stash Data Format
+
+For each stash page:
+
+Byte Offset | Size (bytes) | Description
+------------|:------------:|-------------
+0x00 | 2 | Stash header. Must be "ST" (0x5453 when read as a 16 bit unsigned integer).
+0x02 | 0 or 4 | Flags data (added in PlugY 11.02, doesn't exist in save data from earlier versions). The actual flags can be seen [here](https://github.com/ChaosMarc/PlugY/blob/0116cb44b459ba02832cf8f07092ce4f48aeecdf/PlugY/playerCustomData.h#L19-L27). <br/><br/> It's possible to determine if the flags data exists by checking if the 2 bytes following the next null character == the D2 item header ("JM"); see [the PlugY source code for an example](https://github.com/ChaosMarc/PlugY/blob/0116cb44b459ba02832cf8f07092ce4f48aeecdf/PlugY/InfinityStash.cpp#L258-L264).
+0x02 or 0x06 | At least 1 | Stash page name as a null-terminated string. Can be a maximum of 15 characters + the null terminator (16 total bytes).
+Varies | ... | Beginning of [D2 Inventory data](formats/d2.md#inventory-data-format)
+
+## Sources / Further Reading
+
+* [PlugY save files structures (The Phrozen Keep)](https://d2mods.info/forum/viewtopic.php?f=133&t=31359)
+* [loadStash (PlugY 11.02 Source Code)](https://github.com/ChaosMarc/PlugY/blob/0116cb44b459ba02832cf8f07092ce4f48aeecdf/PlugY/InfinityStash.cpp#L243-L282)