Better document implicit defaults of `labels`/`annotations`

Description

The relevant source is reasonably straight-forward to interpret and unlikely to change:

https://github.com/docker/metadata-action/blob/ed95091677497158a9ff38b314264cd965388d5e/src/meta.ts#L540-L550

NOTE: Presently some of those defaults cannot be easily expressed explicitly (sometimes preferable as visibility for maintainers vs implicit knowledge), but this would be much simpler to document if the following was resolved:

• https://github.com/docker/metadata-action/issues/584
• https://github.com/docker/metadata-action/issues/585
• https://github.com/docker/metadata-action/issues/591

org.opencontainers.image.version in particular has been a common caveat users have tripped up on. More relevant direct documentation on this default could link to the priority attributes section for improved visibility/discovery and thus awareness (instead of encountering it after tags are published). See the last section of my next comment for more info.

Read src/meta.ts at lines 540-550 to confirm the implicit defaults for labels and annotations, then review issues 584, 585, and 591 because they may affect how the defaults can be documented. Update the relevant documentation to make the defaults, especially org.opencontainers.image.version, more visible and link to the priority attributes section.