doc: add missing documentation source files (#1626)

docs/src/manual/algebras/basics.md

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+# Basics
+
+```@meta
+CurrentModule = Hecke
+DocTestSetup = quote
+  using Hecke
+end
+```
+
+## Creation of algebras
+
+See the corresponding sections in ?????
+```@docs
+zero_algebra(::Field)
+```
+
+## Basic properties
+
+```@docs
+base_ring(::Hecke.AbstractAssociativeAlgebra)
+basis(::Hecke.AbstractAssociativeAlgebra)
+```
+
+## Predicates
+
+```@docs
+is_zero(::Hecke.AbstractAssociativeAlgebra)
+is_commutative(::Hecke.AbstractAssociativeAlgebra)
+is_central(::AbstractAssociativeAlgebra)
+```
+
+## Generators
+
+```@docs
+gens(::AbstractAssociativeAlgebra)
+gens_with_data(::AbstractAssociativeAlgebra)
+```
+
+## Center
+
+```@docs
+center(::Hecke.AbstractAssociativeAlgebra)
+dimension_of_center(::Hecke.AbstractAssociativeAlgebra)
+dimension_over_center(::Hecke.AbstractAssociativeAlgebra)
+```

docs/src/manual/algebras/intro.md

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+```@meta
+CurrentModule = Hecke
+```
+
+# [Introduction](@id Algebras)
+
+!!! note
+    The functions described in this section are experimental. While the overall
+    functionality provided will stay the same, names of specific functions or
+    conventions for the return values might change in future versions.
+
+This section describes the functionality for finite-dimensional associative
+algebras (or just *algebras* for short). Since different applications have different requirements, 
+the following types of algebras are implemented:
+- structure constant algebras,
+- matrix algebras,
+- group algebras,
+- quaternion algebras.
+
+These share a common interface encompassing a wide range of functions, which is
+indicated by the use of the type `AbstractAssociativeAlgebra` in the signature.

docs/src/manual/algebras/structureconstant.md

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+# Structure constant algebras
+
+```@meta
+CurrentModule = Hecke
+DocTestSetup = quote
+  using Hecke
+end
+```
+
+## Creation
+
+```@docs
+structure_constant_algebra(::Ring, ::Array{<:Any, 3})
+structure_constant_algebra(::SimpleNumField)
+```
+
+## Structure constant table
+
+```@docs
+structure_constant_table(::StructureConstantAlgebra)
+```