Document the tools changes since 14.0.3

[oryan-block]

Docs for what changed in graphql-java-tools since 14.0.3, so 14.1.0, 14.2.0 and what's on master for the next release.

• Getting started: the io.github.graphql-java-kickstart groupId (changed with the move to the new Maven Central in March) and 14.2.0.
• Defining a schema: naming a schema string and per-source line numbers (#838), resolvers for supertypes and generic classes (#820, #832), Kotlin extension functions as resolver methods (#835), lists on map-backed types (#829), input types sharing a dictionary class (#851) and sealed classes (#854).
• Schema parser options: allowUndeclaredDirectives (#824), a Future has to be a CompletionStage at runtime (#839), null from a wrapper transformer (#836), suspend functions starting on the calling thread (#831) and the CoroutineScope in the GraphQLContext.
• Directives: wirings on the same element see each other's changes (#830), argument defaults (#834), enums only used by directives (#852) and directives on schema (#824).

I left out the fixes that just make things work the way the docs already say they do, like generic input types.

Second commit fixes stale bits on the same pages that were older than this: the snapshot repository (JFrog is long gone, snapshots go to the Maven Central snapshot repository), compile -> implementation in the Gradle snippet, the Maven Central and Travis badges, and the graphql-java directives and field visibility links (both 404).

Built it locally with Hugo 0.167.0 (same as the workflow). No errors, and the new links resolve.

⚠️ Some of this isn't released yet. Should merge after the next graphql-java-tools release, with the version in getting started bumped to it.

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