Add docs versioning

[anna-geller]

Good examples that we like:

• https://docusaurus.io/docs/
• https://opensearch.org/docs/latest/about/

We should ideally create a snapshot of the docs at the time of the release (as part of the release process) and host it under: https://kestra.io/docs/version_number/docs_page

Examples:

• https://kestra.io/docs/concepts/expression/function --> redirect to https://kestra.io/docs/latest/concepts/expression/function?
• Latest docs: https://kestra.io/docs/latest/concepts/expression/function
• Latest docs: https://kestra.io/docs/develop/concepts/expression/function = not published version yet
• Older version: https://kestra.io/docs/0.17.13/concepts/expression/function

Tag main during the release 0.18.2 -- https://kestra.io/docs/0.18.2/concepts/expression/function

To do:

• [ ] Dropdown
• [ ] Adjust the release process to create a docs snapshot at the time of a release
• [ ] Vercel hosting (deployment setup)

[anna-geller]

docs versioning on the website got deprioritized for now, instead we'll work on offering versioned docs directly within the app https://github.com/kestra-io/kestra-ee/issues/1535