[codex] Add sequence feature cookbook

[shenweichen]

Summary

• Add a new Sequence Feature Cookbook covering multi-value, VarLenSparseFeat, DIN/BST history, DSIN session, and TFRecord/Estimator input conventions.
• Link the cookbook from the docs home toctree, Quick-Start, and the multi-value Movielens example.
• Document common answers for sequence-related issues, including padding, maxlen, embedding sharing, hist_ naming, DIN's lack of nested VarLen support, DSIN offline session preprocessing, and dense vector TFRecord inputs.

Why

Several open issues ask overlapping questions about sequence feature preprocessing and model input shapes. A single cookbook gives maintainers one canonical link to use when answering those questions.

Related issues: #521, #515, #492, #478, #375, #369, #346, #302, #254, #230, #204, #169, #143, #109, #75.

Validation

• git diff --check
• git diff --cached --check
• python -m sphinx -q -b html docs/source /tmp/deepctr-docs-sequence-check-2

The Sphinx build succeeded. It still reports existing repository warnings around old autodoc/estimator imports, markdown/docutils formatting, and existing unresolved links; this PR did not address those broader docs warnings.