proposal: Create a dedicated Github repository for Haystack demos (#3695)

* first draft

* add PR number and motivations

* mention HSH

* review feedback

* Update 3695-demo-repository.md

proposals/text/3695-demo-repository.md

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+- Start Date: 2022-12-12
+- Proposal PR: https://github.com/deepset-ai/haystack/pull/3695
+- Github Issue: (if available, link the issue containing the original request for this change)
+
+# Summary
+
+A new git repository is created to host NLP applications showcasing Haystack's features.
+
+# Basic example
+
+A git repository was already created on Github as an example:
+
+https://github.com/deepset-ai/haystack-demos
+
+# Motivation
+
+NLP applications showcasing Haystack's capabilities can be an invaluable learning resource
+for its users, but at this moment we don't fully take advantage of the only one demo we have
+as a documentation source.
+
+This proposal aims at overcoming that limitation in two ways:
+- Define better requirements for a demo application so that users can learn from it.
+- Make it easier to add more demo applications showcasing Haystack.
+
+# Detailed design
+
+Every demo has a descriptive name that will be used as its identifier.
+
+Every demo lives in a dedicated folder named after its identifier at the root of the repo, and
+provides all the resources needed to understand the code, run the application locally or deploy it
+remotely on a server.
+
+Every demo provides a README.md file containing the following information:
+- A brief description of the application and what's its goal.
+- Explicit mention of which NLP use case is implemented: for example "QA", or "Document Retrieval".
+- Detailed instructions about how to run the application locally.
+- Any hardware requirement and the limitations when not provided (for example, a GPU device).
+- How to modify and test the code, and how to contribute changes.
+
+The code of a demo application should be tested whenever possible, and at least some of the
+tests should be able to run in the repo CI system. Every demo has a dedicated Workflow defined
+in a file named after its identifier. The workflow runs only when files in the demo folder are
+modified.
+
+In case the CI is needed for continuous deployment, or for building artifacts, a demo can have
+more than one workflow file defined, named after its identifier plus a descriptive suffix, for
+example: `my_demo_identifier.yml` for tests, `my_demo_identifier_docker.yml` for building a
+Docker image, `my_demo_identifier_deploy.yml` for continuous delivery.
+
+# Drawbacks
+
+- The code of the existing demo would be removed from Haystack and potentially become harder to
+  find for existing contributors.
+- The proposed design dictates a list of new requirements for a demo that will take time to
+  implement.
+
+# Alternatives
+
+- Leave things as they are
+- Implement the design proposal to a subfolder of Haystack's git repository
+
+# Adoption strategy
+
+Adoption will be mostly driven by communicating the changes to the community and monitoring the
+traffic in the new Github repository: interacting with the existing demo will not be affected
+but accessing the code would.
+
+# How we teach this
+
+- A link to the demo repository will be added to the web page of the [demo itself](https://haystack-demo.deepset.ai/).
+- Haystack's README and documentation will mention where to find the code for the demos.
+- [Haystack Home](https://haystack.deepset.ai) will host a whole section dedicated to Haystack demos
+  (detailing the aforementioned section is out of scope for this proposal).
+
+# Unresolved questions
+
+N/A.