docs: restructure navigation into 7 logical tabs

Phase 1 nav-only refactor (no file moves):

**New tab structure:**
- Overview: Getting started, account setup, analytics tools
- Connect Your Data: Integrations, config sheet (renamed from "Data Integrations & Inputs")
- Understand Your Data: Core concepts, attribution (single home), order segmentation
- Use Your Data: Managed BI, MDW, templates, common analyses
- Reference: Data dictionary, table schemas, naming conventions, raw data overviews
- Help: FAQs (by category) + Troubleshooting (separated)
- MTA: Unchanged

**Key changes:**
- Attribution consolidated to single location (was in 3 places)
- FAQs elevated and separated from Troubleshooting
- Reference tab separates "look up" content from "learn" content
- Common Analyses moved to Use Your Data
- Removed duplication per "single location per page" rule

Author: lordhumunguz