docs: add missing alt attributes to documentation images

Add context-aware alt text to 36 images across 20 MDX files flagged
by mint a11y. Descriptions are derived from each image's caption and
surrounding step text. Part of fosrl/docs-v2#116.

manage/access-control/approvals.mdx

@@ -18,13 +18,13 @@ By default, any client configured with valid credentials can connect to an organ
 When device approvals are enabled, the first time a user connects a new device to the organization, the device will be marked as "Pending Approval." An administrator must then review and approve the device in the management console before it can access organization resources.
 
 <Frame>
-  <img src="/images/device_waiting_approval.png" />
+  <img src="/images/device_waiting_approval.png" alt="Device marked pending approval in the Pangolin dashboard" />
 </Frame>
 
 All approvals can also be managed from a central page as they stream in to allow admins to approve or deny devices quickly.
 
 <Frame>
-  <img src="/images/approvals_page.png" />
+  <img src="/images/approvals_page.png" alt="Approvals page listing pending devices in the Pangolin dashboard" />
 </Frame>
 
 ## Enabling Device Approvals

manage/access-control/create-user.mdx

@@ -18,7 +18,7 @@ Because the global user exists and a per‑organization user exists, a user invi
 When removing a user from an organization, their account still exists. To completely delete their account, visit the server admin panel as the server admin and delete the global user in the users table.
 
 <Frame>
-  <img src="/images/users-table.png" centered/>
+  <img src="/images/users-table.png" alt="Users table in the Pangolin dashboard" centered/>
 </Frame>
 
 ### Internal Users

manage/access-control/links.mdx

@@ -45,7 +45,7 @@ This is why the two URLs often look different:
 - The **Access Token Usage** examples use the resource URL directly.
 
 <Frame>
-  <img src="/images/links-access-token-usage.png" />
+  <img src="/images/links-access-token-usage.png" alt="Access token usage examples for a shareable link" />
 </Frame>
 
 ### Query Parameter

manage/access-control/login-page.mdx

@@ -29,7 +29,7 @@ Custom organization authentication pages let you serve the login page at your ow
 - Direct access to the Pangolin management dashboard
 
 <Frame>
-  <img src="/images/org-auth-page.png" />
+  <img src="/images/org-auth-page.png" alt="Organization login page with multiple login methods" />
 </Frame>
 
 ## Configuration
@@ -43,5 +43,5 @@ You need to add a custom domain to your organization first. Free domains (`*.tun
 </Note>
 
 <Frame>
-  <img src="/images/set-org-auth-page-domain.png" />
+  <img src="/images/set-org-auth-page-domain.png" alt="Domain picker for the auth page in Pangolin settings" />
 </Frame>