docs: update 1-3-navigating-the-dashboard.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟡 medium

What changed

Documented the new Revenue & Invoicing overview page (opened by clicking the section header in the sidebar), which surfaces AR aging, invoice status, and upcoming cash-in stats along with quick-create shortcuts for invoices, customers, contracts, and products.

Source PRs

• axolotl#4708 — feat/BLUE 1323/lead to cash category overview

How to use this PR

1. Review the diff against the existing article — focus on accuracy and customer voice.
2. Merge if it reads well. Edit and push to this branch if it needs adjustment.
3. Close without merging if the article doesn't actually need this update.

Once we have a track record of clean drafts, we'll graduate to auto-merge with a delay buffer. For now: human-in-the-loop.