Fix cross-version conflict errors in the "docs" subproject.

[iswoqqe]

Goals

This PR addresses two issues:

• The project cannot be successfully imported in Intellij IDEA with the Scala plugin.
• The sbt command docs/mdoc does not run successfully.

Both currently fail with the error:

[error] Modules were resolved with conflicting cross-version suffixes in ProjectRef(uri("file:/home/iswoqqe/git/inox/"), "docs"):
[error]    org.scala-lang.modules:scala-parser-combinators _3, _2.13

Changes

• The 2.13 version of the parser combinators dependency is now excluded in the "docs" subproject.
  • This makes it possible to import the project, but docs/mdoc still fails with a few different errors, which is why the changes below also are applied.
• Scala version of the "docs" subproject is increased to 3.5.2 (same as root project).
• The mdoc plugin version is increased to 2.6.3 (latest version).

[CLAassistant]

All committers have signed the CLA.

[sankalpgambhir]

Thanks for the PR! Could you mark the PR as not-draft, so the CI can also run? Additionally, have a look at the CLA when you can.

This is probably good reason to test the docs project in the CI as well going forward.