Introduce versioned documentation

[michaelkoepf]

Is your feature request related to a problem? Please describe.
Documentation (i.e., pages that contain version-specific Operator SDK links and information) should be versioned.

Describe the solution you'd like
Use doc versioning feature of docsy.

Steps:

• Find out how to use doc versioning feature (see link above)
• For each release (maybe only up to v4?), check out the state of the website back then and build it [*].
• Replace all absolute links to javaoperatorsdk.io with relative links of the current website version.
• Deploy all past and the current version.
• Adapt CI/CD pipelines.

[*] But take care of absolute links to javaoperatorsdk.io. These need to be replaced first.

The ultimate goal should be to have versioned docs incl. the "latest" version (next release).

Describe alternatives you've considered
n/a

Additional context
n/a

[csviri]

@michaelkoepf thx, yes this would be nice to have; do you plan to do a PR for this? it would great thx

[michaelkoepf]

@csviri in general yes, but this will not happen any time soon i think :( maybe we can label it as good first issue and if somebody stumbles upon it and wants to pick it up, they can go ahead.