Add indices and stats file format specification

Add a specification for a container file format to store indices and
stats for Iceberg tables.

landing-page/content/common/statistics-format-spec.md

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+---
+url: statistics-format
+toc: false
+---
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+
+# Index and statistics file format
+
+This is a specification for the Plain Format for Iceberg Statistics, a file
+format designed to store information such as statistics about data managed in an
+Iceberg table that cannot be stored directly within the Iceberg manifest. A
+statistics file contains arbitrary pieces of information (here called "blobs"),
+along with metadata necessary to interpret them. The blobs supported by Iceberg
+are documented at [Blob types](#blob-types).
+
+## Format specification
+
+A file conforming to the format specification should have the structure as
+described below.
+
+### File structure
+
+The file has the following structure
+
+```
+Magic Blob₁ Blob₂ ... Blobₙ Footer
+```
+
+where
+
+- `Magic` is four bytes 0x80, 0x70, 0x73, 0x83 (short for: Plain Format for
+  Indices and Statistics),
+- `Blobᵢ` is i-th blob contained in the file, to be interpreted by application
+  according to the footer,
+- `Footer` is defined below.
+
+### Footer structure
+
+Footer has the following structure
+
+```
+FooterPayload FooterPayloadSize Reserved FileFormatVersion Magic
+```
+
+where
+
+- `FooterPayload` LZ4-compressed, UTF-8 encoded JSON payload describing the
+  blobs in the file, with the structure described below,
+- `FooterPayloadSize` is a length in bytes of the `FooterPayload` (compressed),
+  stored as 4 byte integer, little-endian,
+- `Reserved` is 4 bytes reserved for future use, currently should be written as
+  0x00, 0x00, 0x00, 0x00,
+- `FileFormatVersion` is a number, stored as 4 byte integer, little-endian,
+- `Magic` four bytes, same as at the beginning of the file.
+
+### Footer Payload
+
+Footer payload bytes is LZ4-compressed, UTF-8 encoded JSON payload representing
+a single `FileMetadata` object.
+
+#### FileMetadata
+
+`FileMetadata` has the following fields
+
+
+| Field Name | Field Type                              | Required | Description |
+| ---------- | --------------------------------------- | -------- | ----------- |
+| blobs      | list of BlobMetadata objects            | yes      |
+| properties | JSON object with string property values | no       | storage for arbitrary meta-information, like writer identification/version
+
+#### BlobMetadata
+
+`BlobMetadata` has the following fields
+
+| Field Name | Field Type             | Required | Description |
+| ---------- | ---------------------- | -------- | ----------- |
+| type       | JSON string            | yes      | See [Blob types](#blob-types)
+| columns    | list of JSON long      | yes      | list of column IDs the blob was computed for
+| offset     | JSON long              | yes      | The offset in the file where the blob contents start. Reader should assume the value can be more than 2^32.
+| length     | JSON long              | yes      | The length of the blob stored in the file
+| compression_codec | JSON string     | no       | See [Compression codecs](#compression-codecs). If omitted, the data is assumed to be uncompressed.
+
+### Blob types
+
+The file format makes no assumptions about the information stored in it. The
+applications using the format need to agree on type of information stored, the
+format of the information and type designators. The list below is not part of
+the file format specification itself, but is provided for readers' convenience.
+
+| Blob type                      | Description |
+| ------------------------------ | ----------- |
+| ndv-long-little-endian         | 8-bytes integer stored little-endian and representing number of distinct values
+| apache-datasketches-theta-v1   | A serialized form of a "compact" Theta sketch produced by the [Apache DataSketches](https://datasketches.apache.org/) library.
+
+### Compression codecs
+
+The compression codec used should be one of: ZSTD, LZ4. The data can also be
+uncompressed. For maximal interoperability, other codecs are not supported.