docs(kits): correct v6 Maven coordinates and source links

[thomson-t]

Background

The Android SDK v6 migration brought mParticle-maintained kits into this
monorepo and changed the Maven coordinates for most kits. Existing
documentation still directs users to retired standalone repositories and v5
artifacts, which can lead them to install outdated integrations.

What Has Changed

• Pin core SDK, kit, and Android plugin examples to version 6.0.0.
• Replace retired kit repository and search links with monorepo source paths
  and direct Maven Central artifact pages.
• Correct published kit READMEs, onboarding paths, and the maintained Branch,
  Braze, and Leanplum sample dependencies.
• Remove unavailable, obsolete, and unsupported kits from the published kit
  list.

Screenshots/Video

N/A — documentation and build example changes only.

Checklist

• Self-review completed
• Tests added or updated
• Tested locally

[cursor]

PR Summary

Low Risk
Changes are limited to Markdown and example build files; no runtime SDK or build logic is modified.

Overview
Updates documentation so Android SDK v6 consumers use the correct Maven Central coordinates and monorepo paths instead of retired v5 android-*-kit artifacts and external integration repos.

Root docs: README.md pins core, Gradle plugin, and kit examples to 6.0.0, refreshes the Maven badge/link, replaces the large legacy kit table with v6 artifacts under kits/, and trims kits that are not part of the maintained v6 list. ONBOARDING.md aligns settings-kits.gradle include examples with the new module paths (e.g. :kits:adjust:adjust-5).

Per-kit docs and samples: Kit READMEs switch dependency lines from com.mparticle:android-*-kit / 5+ to versioned v6 IDs (e.g. adjust-5, braze-41, onetrust), describe sources as monorepo directories, and point Braze example links at in-repo example/ paths. Sample Gradle snippets (Branch, Braze variants, Leanplum) use the same published coordinates.

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