[Doxygen] Layout adjustments (#6196)

* Layout adjustments.

Took 1 hour 54 minutes

Took 2 minutes


Took 6 minutes

* Remove the empty building page.

Took 7 minutes

* Change to @page

Took 11 minutes

* Change to @page again

Took 52 seconds

* Change to @page again again

Took 2 minutes

* Fence the page declaration in CONTRIBUTING.md

Took 8 minutes

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Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

doc/doxygen-extra-pages/developer_documentation/index.md

@@ -0,0 +1,3 @@
+@page developer_reference Developer Reference
+
+- [Contributing](@subpage contributing)

doc/doxygen-extra-pages/index.md

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+@mainpage Cockatrice Documentation
+
+# Welcome
+
+This is the **main landing page** of the Cockatrice documentation.
+
+- Go to the @subpage user_reference page
+- Or check out the @subpage developer_reference

doc/doxygen-extra-pages/user_documentation/index.md

@@ -0,0 +1,4 @@
+@page user_reference User Reference
+
+@subpage search_syntax_help
+@subpage deck_search_syntax_help