docs(pro): document in-app walkthroughs and the App Tour

[blakeaowens]

Description

Adds a documentation page for In-App Walkthroughs, a new DefectDojo Pro beta feature (feature flag walkthroughs, on by default).

The page covers:

• What a walkthrough is: each step outlines one area of the page, dims the rest, and only the outlined area and the step card respond to clicks. Some steps advance on Next, others when the user does the real thing.
• The lightbulb in the lower right corner, which lists the walkthroughs for the current page (This Page) and the ones available anywhere (General), with Take the Tour, Resume Tour, Retake the Tour and Restart.
• The Learn more link on each step card, which opens the matching section of these docs in a new tab.
• The App Tour: Home, the sidebar, the asset list, one asset, its findings and one finding, including what happens when the user opens a different record or navigates away.
• How the App Tour behaves with nothing to open: it skips the steps that open a record, and when the sandbox is turned on its first and last steps point at the sandbox, where an Open the Sandbox button starts the tour again on sample data.
• The first-visit offer on Home (Start, Not Now, Don't Ask Again).
• The two per-user switches on the User Profile page: Offer Walkthroughs Automatically and Show the Walkthrough Lightbulb.
• Turning the feature off instance-wide under Settings > Feature Flags.
• What is stored per user.

The feature's Feature Flags entry links to this page, so it should merge together with the matching Pro change.

Docs only, no code changes.