Improve docs 2

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Pull request overview

This PR focuses on improving Cake’s documentation by clarifying XOR semantics across adapters, refreshing README links, and adding a new Markdown-based API reference under docs/api/.

Changes:

• Clarifies xor_where documentation to state that Cake always emulates “exactly-one” XOR via OR/AND/NOT, and points users to where.xor_parity for MariaDB/MySQL parity XOR.
• Updates README HexDocs links and removes the large “Library Design” diagram section from the README.
• Adds a new API reference documentation set under docs/api/* and registers a new “DIAGRAMS” documentation page in gleam.toml.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/cake/update.gleam	Updates xor_where doc comment to clarify adapter behavior and direct users to xor_parity where appropriate.
src/cake/select.gleam	Updates xor_where doc comment with the same clarified XOR-emulation guidance.
src/cake/delete.gleam	Updates xor_where doc comment with the same clarified XOR-emulation guidance.
README.md	Updates HexDocs URL and removes the in-README “Library Design” diagram section.
gleam.toml	Adds a “DIAGRAMS” docs page entry (currently pointing at docs/diagrams.md).
docs/api.md	Adds a top-level API docs placeholder page.
docs/api/index.md	Adds an API reference landing page and module index.
docs/api/select.md	Adds Markdown API reference for cake/select.
docs/api/insert.md	Adds Markdown API reference for cake/insert.
docs/api/update.md	Adds Markdown API reference for cake/update.
docs/api/delete.md	Adds Markdown API reference for cake/delete.
docs/api/where.md	Adds Markdown API reference for cake/where, including xor vs xor_parity explanation.
docs/api/join.md	Adds Markdown API reference for cake/join (includes some exclusive-join wording issues).
docs/api/fragment.md	Adds Markdown API reference for cake/fragment, including safety guidance.

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