Replace 'headerFooter' by 'standalone'

[abelsromero]

I see in the docs that Ruby code :header_hooteris not the recommended option, but instead should be:standalone`.

So, as in other cases, I think we should:

• [ ] Replace it in main branch: since we can do breaking changes :fireworks:
• [ ] Deprecate in v2.5.8

The only other discrepancy with Asciidoctor Ruby I see are:

• :header_only (in parseHeaderOnly in AsciidoctorJ), not sure about this, should we rename it?
• timings, which is missing and I'd also like to add, the cli already has some work done. Will create another issue for that.

[abelsromero]

Haven't mentioned, but my ideas with recent changes it:

1. Get v3.0.0 out! Then port similar changes to the maven-plugin too and release it.
2. Be able to provide a v2.5.x for a smother transition. Even if we release at the same time as v3.0.0, people should be able to use 2.5.8 first and don't miss much.