Generate JSON schema index for Schemastore

[ZanSara]

Related to #2110.

Proposed changes:

• As noted in k3d: reference versions schema in k3d repo SchemaStore/schemastore#2057 (comment), Schemastore supports schema indices hosted remotely. They will allow us to be in control of the versioning of our schemas without the need to submit a PR to them every time we release a new schema version.
• Said index is generated in the CI and will always have at most one rc version in it (to avoid pollution). They're unstable anyway (they get overwritten at every push to master) so I don't think they should stay in the index once they become obsolete.
• This change also fixes the (currently failing) CI 😅

Status:

• First draft (up for discussions & feedback)
• Final code
• Added tests
• Updated documentation (the release process especially: https://www.notion.so/deepsetai/PyPi-release-14499451e0484aff949ae7451ddf015b)
• Add catalog entry for Haystack pipelines SchemaStore/schemastore#2090 also updated accordingly.

[julian-risch]

The code-and-docs-check test is failing. Other wise looks good to me. I didn't check each of the 2590 lines of json-schemas/haystack-pipeline-1.2.0rc0.schema.jsonthough. 😉
In the issue description, the list item "Updated documentation" is still open. Feel free to merge once this is addressed.
I was thinking a bit about whether to remove any existing (unstable) rc versions from the schema index. Makes sense to me. 👍