docs: add API documentation links for Pixels, Signals, Signatures, Shape, SpectralImage, and SpectralImageSet classes

Author: janezlapajne

docs/concepts/entities.md

@@ -26,6 +26,9 @@ The class structure is depicted in the following diagram:
 
 ## Pixels
 
+??? api "API Documentation"
+    [`siapy.entities.Pixels`][siapy.entities.Pixels]<br>
+
 The `Pixels` class represents spatial coordinates within spectral image, providing a container for *(x, y)* coordinate pairs. It uses pandas DataFrame internally for storage, enabling high-performance operations. The class provides multiple initialization methods and conversion functions to work with different data representations (i.e. DataFrames, list, arrays)
 
 ```python
@@ -34,6 +37,9 @@ The `Pixels` class represents spatial coordinates within spectral image, providi
 
 ## Signals
 
+??? api "API Documentation"
+    [`siapy.entities.signatures.Signals`][siapy.entities.signatures.Signals]<br>
+
 The `Signals` class stores spectral data for each pixel in a pandas DataFrame, allowing you to use any column names you choose (e.g. "band_1", "nir", "red_edge"). You can initialize it from a DataFrame, lists, dicts or NumPy arrays.
 
 ```python
@@ -44,6 +50,9 @@ However, direct initialization of `Signals` is typically not necessary in practi
 
 ## Signatures
 
+??? api "API Documentation"
+    [`siapy.entities.Signatures`][siapy.entities.Signatures]<br>
+
 The `Signatures` class represents spectral data collections by combining spatial coordinates (`Pixels`) with their corresponding spectral values (`Signals`). It provides a unified container that maintains the spatial-spectral relationship, allowing for analysis of spectral information at specific image locations. Internally, the data is stored as pandas DataFrames for efficient operations and indexing.
 
 `Signatures` can be initialized in multiple ways. The explicit approach creates each component separately before combining them, providing clarity about the composition:
@@ -66,6 +75,9 @@ Both approaches yield equivalent results when initialized with the same data. Yo
 
 ## Shape
 
+??? api "API Documentation"
+    [`siapy.entities.Shape`][siapy.entities.Shape]<br>
+
 The `Shape` class represents geometric shapes that can be associated with images, such as points, lines, and polygons.
 
 ```python
@@ -74,6 +86,9 @@ The `Shape` class represents geometric shapes that can be associated with images
 
 ## Spectral Image
 
+??? api "API Documentation"
+    [`siapy.entities.SpectralImage`][siapy.entities.SpectralImage]<br>
+
 A `SpectralImage` is the primary container for spectral image data. It's a generic class that can wrap different image backends, allowing you to work with various file formats through a unified interface.
 
 ### Image Initialization Options
@@ -112,6 +127,9 @@ For specialized file formats or custom processing needs, you can extend the Imag
 
 ## Spectral Image Set
 
+??? api "API Documentation"
+    [`siapy.entities.SpectralImageSet`][siapy.entities.SpectralImageSet]<br>
+
 The `SpectralImageSet` class manages a collection of spectral images.
 
 ```python