Add sidebar documentation structure

Adds a new file defining the wiki sidebar structure, including sections for Theory, Usage commands, and Formats to guide users on various scenarios.

_Sidebar.md

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+# Wiki sidebar
+
+- [[index|Home]]
+- [[installation|Installation]]
+## Theory
+  - [[theory-kmers_and_superkmers|Kmers and super-kmers]]
+  - [[theory-encoding|DNA encoding]]
+  - [[theory-entropy_filter|Low-complexity kmer filter]]
+  - [[theory-minimizer_selection|Minimizer selection]]
+  - [[theory-indexing_architecture|Partitioning and indexing architecture]]
+## Usage
+  - [[usage-superkmer|superkmer]]
+  - [[usage-index_command|index]]
+  - [[usage-merge|merge]]
+  - [[usage-filter|filter]]
+  - [[usage-select|select]]
+  - [[usage-query|query]]
+  - [[usage-dump|dump]]
+  - [[usage-annotate|annotate]]
+  - [[usage-phylo|phylo]]
+  - [[usage-unitig|unitig]]
+  - [[usage-estimate|estimate]]
+  - [[usage-convert|convert]]
+  - [[usage-utils|utils]]
+  - [[usage-pack|pack]]
+  - [[usage-predicates|Predicates and taxonomy paths]]
+## Formats
+  - [[formats-index_layout|Index construction and on-disk layout]]
+- [[architecture|Architecture notes]]