[PATCH proxmox-backup v2] 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 v1: Thanks @Fabian
  * move the formatting below  .ppxar formatting.
  * add description explaining the variable length encoding.
  * seperate the catalog data contents into its own table and update the
    root start offset description.
  * add more explanation about `offset_back`.

 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 | 89 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 88 insertions(+), 1 deletion(-)

diff --git a/docs/file-formats.rst b/docs/file-formats.rst
index ed3250d11..04d0406a7 100644
--- a/docs/file-formats.rst
+++ b/docs/file-formats.rst
@@ -55,6 +55,94 @@ All headers are stored as little-endian.
    * - ``PAYLOAD_TAIL_MARKER``
      - header of ``[u8; 16]`` consisting of type hash and size;
        marks end
+
+.. _pcat1-format:
+
+Proxmox Catalog File Format (``.pcat1``)
+----------------------------------------
+
+The asterisk notation indicates variable-length integers:
+
+* **u64\***: Unsigned variable-length integer. Each encoded byte is 8 bits wide. Bit 7, the most significant bit, is the continuation
+  flag. Bits 0 through 6 carry value bits. The first encoded byte stores the lowest order 7 value bits, the second encoded byte
+  stores the next 7 value bits, and so on.
+
+* **i64\***: Signed variable-length integer. Each nonzero encoded byte is 8 bits wide. Bit 7, the most significant bit, is the
+  continuation flag. Bits 0 through 6 carry value bits. The first encoded byte stores the lowest-order 7 value bits, the second
+  encoded byte stores the next 7 value bits, and so on. Non-negative values end with a byte whose continuation flag is clear.
+  Negative values end with an additional 0x00 terminator byte.
+
+.. list-table::
+   :widths: auto
+
+   * - ``MAGIC: [u8; 8]``
+     - ``[145, 253, 96, 249, 196, 103, 88, 213]``.
+   * - [Catalog Data]
+     - Raw catalog data. This includes TABLE_LENGTH, ENTRY_COUNT and ENTRIES. See ``Catalog Data`` :ref:`contents <catalog-data>`.
+   * - ...
+     - Further concatenation of catalog data. 
+   * - ``ROOT_START_OFFSET: u64``
+     - Points at the start of the '/' catalog entry. Stored as **little-endian**.
+
+Catalog Data:
+
+.. _catalog-data:
+
+.. list-table::
+   :widths: auto
+
+   * - ``TABLE_LENGTH: u64*``
+     - Total length of the contents inside the Directory.
+   * - ``ENTRY_COUNT: u64*``
+     - Number of entries present inside the Directory.
+   * - ``ENTRIES``
+     - Concatenated ``ENTRY`` structures (see the ``ENTRY`` :ref:`layout <entry-layout>` .) Repeated ``ENTRY_COUNT`` times.
+
+.. _entry-layout:
+
+Except for files and directories, no additional payload is stored beyond the name and its length inside an ENTRY.
+The ``offset_back`` stores the backward byte distance from the start of the parent directory table to the start of the
+referenced child directory table. The reader can then compute the start offset of the child directory table as
+``parent_table_start - offset_back``.
+  
+.. 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
+
 .. _data-blob-format:
 
 Data Blob Format (``.blob``)
@@ -103,7 +191,6 @@ encrypted data:
    * - ``TAG: [u8; 16]``
    * - ``Data: (max 16MiB)``
 
-
 .. _fixed-index-format:
 
 Fixed Index Format  (``.fidx``)
-- 
2.47.3