chore(docs): document direct access sharing

[sdolidze]

Closes: https://linear.app/lightdash/issue/SPK-1462/document-direct-access-for-customers-and-internal-teams

Description:

Document how project members share individual dashboards, saved charts, saved SQL charts, and data apps without sharing their containing space. Add a canonical Direct Access guide covering additive roles, content boundaries, discovery, lifecycle behavior, and the access API, with links from the existing role and sharing pages and a content-as-code policy reference.

Direct dashboard grant
  ├── Dashboard and its owned charts
  └── No access granted to containing space or independent content

Includes five screenshots captured through Chrome DevTools on Lightdash 2.173.1 (d1d0f014d2), using demo data: user sharing, group sharing, role selection, removing access, and Shared with me.

Validation: repository documentation validator passes with zero errors, Mintlify reports no broken links, and git diff --check passes. All five images load in the local preview; desktop and 390px mobile layouts were inspected without horizontal overflow.

The dashboard flow was exercised in separate administrator and recipient browser sessions: create user/group grants, change a role, remove the user grant while retaining group access, discover and open the dashboard through Shared with me, and remove all remaining grants. The recipient received 200 for the granted dashboard and 403 for its containing space; after revocation, the dashboard returned 403 and disappeared from Shared with me.

Draft pending the remaining resource-specific acceptance checks, Product/engineering and Support review, and coordination with feature enablement. This PR covers Mintlify; the ticket's internal Notion runbook remains outstanding, so the closing link must be revisited if that work is still open at merge time.

[linear-code]

SPK-1462

[mintlify]

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[github-actions]

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