GH-533: Add ALP (Adaptive Lossless floating-Point) encoding specification

[prtkgaur]

Add the encoding specification for ALP (encoding value 10) to Encodings.md. ALP compresses FLOAT and DOUBLE columns by converting values to integers via decimal scaling, then applying Frame of Reference encoding and bit-packing. Values that cannot be losslessly round-tripped are stored as exceptions.

• Closes [Proposal] Add ALP encoding support in parquet file format #533

See rendered preview here: https://github.com/prtkgaur/parquet-format/blob/alpEncoding/Encodings.md#adaptive-lossless-floating-point-alp--10

The spec covers:

• Page layout: 7-byte header, offset array, compressed vectors
• Vector format: AlpInfo, ForInfo, packed values, exception data
• Encoding math: two-step multiplication for cross-language consistency
• Parameter selection, exception detection, and decoding steps

Based on the paper "ALP: Adaptive Lossless floating-Point Compression" (Afroozeh and Boncz, SIGMOD 2024). Wire format matches the C++ Arrow and Java parquet-java implementations.

Rationale for this change

What changes are included in this PR?

Do these changes have PoC implementations?