Extend vlp by adding a processor for xref:latest or xref:dev

[jhkrug]

I've put in a hook for when processing adoc files.

We can use cross-component xrefs with links to the latest and dev versions. Like:

- xref:latest@rancher-manager:en:/about-rancher/overview.adoc#_run_kubernetes_everywhere[link to rancher-manager]
- xref:dev@admission-controller:en:introduction.adoc#_benefits_and_value[link to dev]
- xref:latest@admission-controller:en:introduction.adoc#_benefits_and_value[link to latest]

Before you would have to use an actual version, like:

- xref:v2.12@rancher-manager:en:/about-rancher/overview.adoc#_run_kubernetes_everywhere[link to rancher-manager]
- xref:1.31@admission-controller:en:introduction.adoc#_benefits_and_value[link to dev]
- xref:1.30@admission-controller:en:introduction.adoc#_benefits_and_value[link to latest]

The hook replaces xref:latest and xref:dev with the xref:actual actual latest and dev versions determined while doing the processing to construct the latest and dev symlinks.

It was always possible to use a link macro to a URL with latest or dev, but there is no build-time link checking.

[jhkrug]

Back to draft while I check:

Does it cope with the situation where the only version is latest (and maybe a dev). So, they are real versions, and not just going to be symlinks. May need a little more exception processing.