docs: add Sourcey-generated docs and llms.txt

[IvanchitorJR]

Summary

• add a reproducible Sourcey configuration that imports the existing MkDocs navigation
• add the generated llms.txt under docs/, the existing MkDocs source directory
• link the live Sourcey docs preview and the repository-owned docs/llms.txt from the README

Placing the generated file at docs/llms.txt means KeyBERT's existing MkDocs deployment will copy it to the project-owned https://maartengr.github.io/KeyBERT/llms.txt path after merge. The current Sourcey-generated preview is available at https://ivanchitorjr.github.io/KeyBERT/ so maintainers can inspect the complete 17-page output before adopting it.

The preview was generated from KeyBERT commit 4d3230bba897f316fad36eafa7994d9ff8b1b9e5 with Sourcey 3.6.5. The same adapter can be wired into the project's Pages workflow without changing documentation sources.

Reproduce

npm install --no-save --package-lock=false sourcey@3.6.5
npx sourcey build --config sourcey.config.ts --output dist

The generated dist/llms.txt is byte-for-byte identical to docs/llms.txt at SHA-256 4a387269f407a2314d9532484aef0b0a404e78597026830975b0f7815d0a486a.