Split vale rules into two style sets

Problem to solve

As highlighted in gitlab-org/modelops/ai-model-validation-and-research/ai-evaluation/prompt-library!583 (comment 2003621447), people want to use our vale rules in other projects. While we haven't created an easily distributable package for our vale rules yet, we could consider making this a bit easier to port to other projects.

A "simple" (logically) change we could do would be to split our gitlab style into two:

• gitlab-base: Basic technical writing rules that can apply to all docs (mostly) universally. Everything except the rules listed in gitlab-docs below.
• gitlab-docs: Vale rules that ensure documentation published at docs.gitlab.com renders properly：
  • doc/.vale/gitlab/AlertBoxStyle.yml
  • doc/.vale/gitlab/Badges-Offerings.yml
  • doc/.vale/gitlab/Badges-Tiers.yml
  • doc/.vale/gitlab/HistoryItems.yml
  • doc/.vale/gitlab/HistoryItemsOrder.yml
  • doc/.vale/gitlab/TabsLinks.yml
  • doc/.vale/gitlab/RelativeLinks.yml (added)
  • doc/.vale/gitlab/InternalLinksCode.yml (added)

Product documentation that we'd need to determine if they should be gitlab-base level, or only for gitlab-docs:

• doc/.vale/gitlab/ReadingLevel.yml
• doc/.vale/gitlab/CurrentStatus.yml
• doc/.vale/gitlab/FutureTense.yml
• doc/.vale/gitlab/Simplicity.yml

Further details

We'd probably want to expands the docs, for example at https://docs.gitlab.com/ee/development/documentation/testing/vale.html, to explain how to port this to other projects.