[Docs] Disambiguate headings (for the general handbook)

docs/admin/00_index.rst

-Configuration and administration
-================================
+Setup of the AlekSIS instance and the AlekSIS core
+==================================================
 
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docs/appendix/00_index.rst

-Appendix
-========
+Appendices of AlekSIS-Core
+==========================
 
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docs/dev/00_index.rst

-Development of AlekSIS apps
-===========================
+Development of AlekSIS apps using the AlekSIS core
+==================================================
 
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docs/preamble/00_index.rst

-Preamble
-========
+Introduction to AlekSIS and the AlekSIS core
+============================================
 
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docs/ref/00_index.rst

-API and models reference
-========================
+Core API and models reference
+=============================
 
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docs/user/00_index.rst

-End-user documentation
-======================
+Using basic features of the AlekSIS Core
+========================================
 
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