[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.

Fixes: https://bugzilla.proxmox.com/show_bug.cgi?id=5748
Signed-off-by: Shan Shaji <s.shaji@proxmox.com>
---
 changes since RFC: Thanks @Christian Ebner
 - Changed from graph to table layout.
 - Explicity mention about the variable length types. 
 - Denoted the root start offset as u64.
 - Add text description for each field.
 
 docs/file-formats.rst | 65 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/docs/file-formats.rst b/docs/file-formats.rst
index ed3250d11..52cb20ee0 100644
--- a/docs/file-formats.rst
+++ b/docs/file-formats.rst
@@ -23,6 +23,71 @@ overhead introduced by the file payloads.
 
 .. graphviz:: meta-format-overview.dot
 
+.. _pcat1-format:
+
+Proxmox Catalog File Format (``.pcat1``)
+----------------------------------------
+
+The asterisk notation indicates variable-length encoding: u64* denotes an unsigned variable-length encoded integer, while i64*
+denotes a signed variable-length encoded integer.
+
+.. list-table::
+   :widths: auto
+
+   * - ``MAGIC: [u8; 8]``
+     - ``[145, 253, 96, 249, 196, 103, 88, 213]``.
+   * - ``TABLE_LENGTH: u64*``
+     - Total length of the contents inside the Directory.
+   * - ``ENTRY_COUNT: u64*``
+     - Count of entries present inside the Directory.
+   * - ``ENTRIES``
+     - Concatinated ``ENTRY`` structures (see the ``ENTRY`` :ref:`layout <entry-layout>` .) Repeated ``ENTRY_COUNT`` times.
+   * - ...
+     - Further concatination of sequential directory blocks (Table Length, Entry Count and Entries).
+   * - ``ROOT_START_OFFSET: u64``
+     - Root marker (/) start offset. Stored as **little-endian**
+
+.. _entry-layout:
+
+Except for files and directories, no additional payload is stored beyond the name and its length inside an ENTRY.
+
+.. list-table::
+   :widths: auto
+
+   * - ``ENTRY_TYPE: u8``
+     - The type of entry (see entry :ref:`types <entry-types>`).
+   * - ``NAME_LENGTH: u64*``
+     - Length of file or directory name. Stored as variable length encoded u64.
+   * - ``NAME: [u8]``
+     - The raw UTF-8 string bytes of the name.
+   * - ``PAYLOAD``
+     - Bytes based on ``ENTRY_TYPE``
+
+       * **Directory ('d')**: ``offset_back: u64*`` (Child-to-Parent offset distance)
+       * **File ('f')**: ``size: u64*`` followed by ``mtime: i64*``
+.. _entry-types:
+
+Entry Types:
+
+.. list-table::
+   :widths: auto
+
+   * - ``d``
+     - Directory
+   * - ``f``
+     - File
+   * - ``l``
+     - Symlink
+   * - ``h``
+     - Hardlink
+   * - ``b``
+     - Block device
+   * - ``c``
+     - Char device
+   * - ``p``
+     - FIFO
+   * - ``s``
+     - Socket
 .. _ppxar-format:
 
 Proxmox File Archive Format - Payload (``.ppxar``)
-- 
2.47.3