[RFC PATCH proxmox-backup] fix #5748: docs: add `catalog.pcat1` format specification

[Shan Shaji <s.shaji@proxmox.com>]

Earlier proxmox catalog file format specification was not present
inside the docs. fixed it by adding format specification under file
formats.

Signed-off-by: Shan Shaji <s.shaji@proxmox.com>
---
 Sending this as an RFC because I am not sure whether the names I used make sense.

 I also marked `TABLE_LENGTH`, `ENTRY_COUNT`, `NAME_LENGTH`, `offset_back`, `size`, and
 `mtime` as 64-bit values. However, AFAIU, when these values are written to the file,
 they are encoded as variable length byte sequences. For example, if `NAME_LENGTH` is
 greater than 127, it will be encoded a multiple `u8` values. So, Is it okay to specify
 the type in logical size?

 docs/catalog-format-overview.dot | 45 ++++++++++++++++++++++++++++++++
 docs/file-formats.rst            |  6 +++++
 2 files changed, 51 insertions(+)
 create mode 100644 docs/catalog-format-overview.dot

diff --git a/docs/catalog-format-overview.dot b/docs/catalog-format-overview.dot
new file mode 100644
index 000000000..a9ec64ad3
--- /dev/null
+++ b/docs/catalog-format-overview.dot
@@ -0,0 +1,45 @@
+digraph structs {
+    graph [
+        rankdir = "LR"
+        fontname="Helvetica"
+    ];
+
+    node [
+        fontsize = "16"
+        shape = "record"
+    ];
+
+    catalog [
+        label = "catalog.pcat1"
+    ];
+
+    catalog_payload [
+        label = "<magic> CATALOG_MAGIC: [u8; 8]\l|<blocks> \{DIRECTORY_BLOCK\}*\l|ROOT_DIRECTORY_OFFSET: [u8; 8]\l"
+    ];
+
+    directory_block [
+        label = "<table_length> TABLE_LENGTH: u64\l|<entry_count> ENTRY_COUNT: u64\l|<entries> \{DIRECTORY_ENTRY\}*\l"
+    ];
+
+    directory_entry [
+        label = "<entry_type> ENTRY_TYPE: u8\l|NAME_LENGTH: u64\l|NAME: [u8]\l|<attr> ATTRIBUTE_PAYLOAD\l"
+    ];
+
+    attribute_payload [
+        label = "{{DIRECTORY_PAYLOAD|offset_back: u64}|{FILE_PAYLOAD|size: u64\nmtime: i64}|{OTHER_PAYLOAD|no extra payload}}"
+    ];
+
+    entry_types [
+        label = "'d' directory|'f' file|'l' symlink|'h' hardlink|'b' block device|'c' char device|'p' fifo|'s' socket"
+    ];
+
+    catalog -> catalog_payload:magic;
+
+    catalog_payload:blocks -> directory_block:table_length;
+
+    directory_block:entries -> directory_entry:entry_type;
+
+    directory_entry:attr -> attribute_payload;
+
+    directory_entry:entry_type -> entry_types;
+}
diff --git a/docs/file-formats.rst b/docs/file-formats.rst
index ed3250d11..f67b1b23c 100644
--- a/docs/file-formats.rst
+++ b/docs/file-formats.rst
@@ -1,5 +1,11 @@
 File Formats
 ============
+.. _pcat1-format:
+
+Proxmox Catalog File Format (``.pcat1``)
+----------------------------------------
+
+.. graphviz:: catalog-format-overview.dot
 
 .. _pxar-format:
 
-- 
2.47.3