docs: link the pre-install checklist and IaaS roles pages

Two maintained documentation pages were unreachable: they carried real
content but had no sidebar entry and no inbound links, an oversight from
the subtree merges that assembled the consolidated sidebar.

Wire them in rather than delete them:

- docs/01-getting-started/preinstall-checklist.md is added as a
  top-level entry, directly after the landing page, since it is
  planning material meant to be read before any layer-specific docs.
- docs/05-iam/iaas-roles.md is added under the Identity and Access
  Management category, alongside the federation setup page, as the
  reference for the IaaS-layer roles an SCS-compliant cloud offers.

Both doc ids resolve after Docusaurus strips the numeric directory
prefixes (01-getting-started, 05-iam). The sidebar module still parses
cleanly.

DocImpact
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jan Klare <klare@osism.tech>

Author: jklare

sidebarsDocs.js

@@ -4,6 +4,7 @@
 const sidebarsDocs = {
   docs: [
     'index',
+    'getting-started/preinstall-checklist',
     {
       type: 'category',
       label: 'IaaS Layer',
@@ -333,6 +334,7 @@ const sidebarsDocs = {
             'iam/SCS-example-setup-configuration-description'
           ]
         },
+        'iam/iaas-roles',
         'iam/intra-SCS-federation-setup-description-for-osism-doc-operations'
       ]
     },