chore: set up Renovate for dependency updates [MEC-3447]

[contentful-renovate-rollout]

Sets up Renovate as the dependency-update tool for this repository, part of an org-wide rollout.

Depending on this repo's starting state, this PR does some of the following:

• adds a repo-local renovate.json extending the Contentful shared preset
• for repos that used Dependabot: ports repo-specific behavior (PR limits, ignores, grouping) into local packageRules where still needed, and removes Dependabot-only automation and secret policies
• updates docs that referenced Dependabot

Manual follow-up (not handled by this PR):

• close stale Dependabot PRs and branches once Renovate is active (for repos that used Dependabot)

Migration Confidence: 🔴 Low

Summary

A renovate.json was added at the repo root extending the correct shared preset (local>contentful/renovate-config). However, the file also includes speculative packageRules for Terraform providers and Terraform patch updates. This is a Java/Maven SDK project (contentful-management.java) with no Terraform infrastructure files present, so these rules are entirely unjustified by the repo shape. Per migration assessment rules, adding speculative Renovate tuning that nothing justified is a low-confidence signal. No Dependabot config existed to remove, so this is a fresh Renovate install, but the extra rules should not be there.

Files requiring attention

The following files had edge cases or required judgment during migration and should be reviewed carefully:

• renovate.json