docs: add OAuth M2M auth as recommended option for Databricks

Summary

Updates the Databricks integration docs to document the new OAuth machine-to-machine (M2M) authentication method, positioning it as the recommended option. Personal access tokens are preserved but marked as legacy.

Two files changed:

• create_service_principal.mdx — Restructured step 6 into two credential options: OAuth secret generation (recommended) and PAT creation (legacy). Old steps 6+7 merged; step 8 renumbered to 7.
• databricks.mdx (integration snippet) — Split form fields into common fields and auth-method-specific sections. OAuth (M2M) listed first with <Info> callout; Access token listed second with <Warning> callout advising migration.

Three {/* TODO */} screenshot placeholders are included where new screenshots are needed.

Review & Testing Checklist for Human

• Add screenshots before merging — Three TODO placeholders need real screenshots: (1) Elementary form with OAuth selected, (2) Elementary form with Access token selected, (3) Databricks service principal Secrets tab with "Generate secret" button
• Verify Mintlify rendering — Check that the nested Option A / Option B content under step 6 in create_service_principal.mdx renders correctly inside the numbered list (indented markdown under list items can be fragile)
• Test cross-reference links — The [step 5](#create-service-principal) and [step 7](#create-service-principal) anchor links in databricks.mdx reference the imported snippet; verify these resolve correctly in the rendered page
• Review copy/tone — Confirm the Info and Warning callout text accurately represents the product positioning for OAuth vs legacy tokens

Notes

• Companion feature PR: elementary-internal#5420
• Requested by: @haritamar
• Devin session