Improve "Creating Custom Document Stores" documentation

[dfokina]

• Add a "to-do" checklist
• See Add PGVector DocumentStore haystack-core-integrations#142 (comment) for additional useful information

Tasks

Beta Give feedback

1. How we expect to find the document store: e.g.: We expect them in a separate repo, with XYZ structure, and an accompanying package distributed on PyPI
2. Our recommendations on package naming
3. The naming convention of Haystack components, in particular retrievers
4. How we expect them to handle secret management
5. How we expect them to handle device management (if needed)
6. What they should do for serialization
7. DocumentStoreBaseTests usage
8. Prepare a public repository
9. Release the package
10. Draft a documentation page following the draft.md in the haystack-integrations repo and create a PR.
11. Optionally but very useful, host an api docs for your document store
12. We could also give exemplary repos in the doc IMO. For example, the Neo4j one is amazing, and has its own docs etc.

[TuanaCelik]

@masci here are a list of things I see as missing in the current docs, and some others that would be useful in discussions with document store providers that we speak to:

• How we expect to find the document store: e.g.: We expect them in a separate repo, with XYZ structure, and an accompanying package distributed on PyPI
• Our recommendations on package naming
• The naming convention of Haystack components, in particular retrievers
• How we expect them to handle secret management
• How we expect them to handle device management (if needed)
• What they should do for serialization

Following that, instructions on how to get featured. I see this as a todo list:

• Prepare a public repository
• Release the package
• Draft a documentation page following the draft.md in the haystack-integrations repo and create a PR.
• Optionally but very useful, host an api docs for your document store

We could also give exemplary repos in the doc IMO. For example, the Neo4j one is amazing, and has its own docs etc.

[anakin87]

Also adding some suggestions on how to develop the document store using DocumentStoreBaseTests might help.
deepset-ai/haystack-core-integrations#142 (comment)

[masci]

Template repository is up to date now.