Update Gradle config

[aSemy]

Preparation for #192

• This PR breaks up the Gradle config into smaller convention plugins. Each convention plugin can be applied to each subproject as required. This helps with organising build configuration, and only applying it as required. All of the config was copy&pasted from the root build.gradle.kts, so nothing should be different.

• I enabled some Gradle caching options. This should speed up build speeds, and when developing locally (for example, when switching branches).

• I adjusted some of the Gradle build config which seemed unusual to me. I changed them to be more conventional - which could be a mistake! I'll make notes of these so you can review them.

• I changed the name of the Maven POMs to include the subproject name, prefixed by skrape{it}.

• This PR includes the README updates #193

• I updated Gradle to 7.5

• I merged the 3 .gitignores into one

WIP:

• [x] Confirm that the dependency update plugins still work as expected
• [x] Confirm that detect still works
• [x] Confirm Dokka still works
• [x] Double check I've copied the signing variables correctly

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Okay, all done!

Changes of note:

• I added a project-local Maven repo, so I can test that the Maven deployment still works and the signatures are generated as expected. Just run ./gradlew publishAllPublicationsToLocalProjectDirRepository
• I added the Dokka docs to the Javadoc jar
• I enabled Gradle caching (see gradle.properties). You might see some configuration cache warnings like

  265 problems were found storing the configuration cache, 75 of which seem unique.
  - Task `:assertions:dokkaJavadoc` of type `org.jetbrains.dokka.gradle.DokkaTask`: cannot serialize object of type 'org.gradle.api.internal.artifacts.configurations.DefaultConfiguration', a subtype of 'org.gradle.api.artifacts.Configuration', as    these are not supported with the configuration cache.

  but they're just warnings. If they're an issue, then set org.gradle.unsafe.configuration-cache=false. Note that the build cache is stable, that shouldn't cause issues.

• I'm not able to verify that Nexus publishing still works, so please check!

[christian-draeger]

Really really big thanks for the great work 🎉👌 This will help to evolve the library a lot 🙂