Rename category "vision" to "strategy"

The handbook differentiates between vision (10y) and strategy (3y), but we still use Vision in a number of places. Now we use it in one less!

Specifically, this change:

• updates terminology in categories.yml
• updates partial used in Stage direction pages https://jramsay-strategy-not-vision.about.gitlab-review.app/direction/create/#categories
• updates partial used in Categories page https://jramsay-strategy-not-vision.about.gitlab-review.app/handbook/product/categories/#dev-section
• updates partial used in Maturity page (strategy link is a fallback for the marketing URL)