Improve the documentation about BLOBs

### Backlog
- [ ] Rework the documentation about BLOBs. Currently, it is scattered between `docs/blobs.rst`, `docs/by-example/http.rst`, and `docs/by-example/blob.rst`, and apparently outlines different access variants or stages. Let's sort this out.

_Originally posted by @amotl in https://github.com/crate/crate-python/issues/448#issuecomment-1343332778_

并排阅读 docs/blobs.rst、docs/by-example/http.rst 和 docs/by-example/blob.rst，首先确定每个文档所描述的不同访问变体或阶段。重新整理这三个部分，使其对 BLOB 处理形成一致的说明，不再留下相互冲突的指导。

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