Backport PR scverse#1941: Add linkable labels to API docs

Author: flying-sheep

docs/api.md

@@ -13,6 +13,8 @@ The central class:
    AnnData
 ```
 
+(combining-api)=
+
 ## Combining
 
 Combining {class}`AnnData` objects.
@@ -25,6 +27,8 @@ See also the section on concatenation.
    concat
 ```
 
+(reading-api)=
+
 ## Reading
 
 Reading anndata’s native formats `.h5ad` and `zarr`.
@@ -66,6 +70,8 @@ You might have more success by assembling the {class}`AnnData` object yourself f
    io.read_umi_tools
 ```
 
+(writing-api)=
+
 ## Writing
 
 Writing a complete {class}`AnnData` object to disk in anndata’s native formats `.h5ad` and `zarr`.
@@ -173,6 +179,8 @@ Types used by the former:
    experimental.StorageType
 ```
 
+(errors-api)=
+
 ## Errors and warnings
 
 ```{eval-rst}
@@ -182,6 +190,8 @@ Types used by the former:
    ImplicitModificationWarning
 ```
 
+(settings-api)=
+
 ## Settings
 
 ```{eval-rst}
@@ -192,6 +202,8 @@ Types used by the former:
    settings.override
 ```
 
+(types-api)=
+
 ## Custom Types/Classes for Readable/Writeable Elements
 
 ```{eval-rst}