ERIERL-1251 warning on doc build

[kvakvs]

Fixes additional problem found with the ticket
The ticket reporter confirmed that make docs work as they want

1. The applications, skipped by configure are written into lib/SKIP-APPLICATIONS.
2. The make docs scripts read it and use words as skip list for docs building.
3. The missing apps links in doc side bar menu are pointing to placeholder pages which now contain a line explaining that the app was skipped.
4. If all applications are built correctly and SKIP-APPLICATIONS does not exist, the warning is now not displayed, and the file contents is assumed to be an empty list

[github-actions]

CT Test Results

  1 files   11 suites   2m 59s ⏱️
 95 tests  91 ✅ 4 💤 0 ❌
111 runs  107 ✅ 4 💤 0 ❌

Results for commit c922ad2.

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