docs/stdlib: write an introductory section.

Co-authored-by: Wanda <[email protected]>
Co-authored-by: mcclure <[email protected]>

docs/stdlib.rst

@@ -1,17 +1,21 @@
 Standard library
 ################
 
-.. todo::
+The :mod:`amaranth.lib` module, also known as the standard library, provides modules that falls into one of the three categories:
 
-   Write this section.
+1. Modules that will used by essentially all idiomatic Amaranth code, and are necessary for interoperability. This includes :mod:`amaranth.lib.enum` (enumerations), :mod:`amaranth.lib.data` (data structures), and :mod:`amaranth.lib.wiring` (interfaces and components).
+2. Modules that abstract common functionality whose implementation differs between hardware platforms. This includes :mod:`amaranth.lib.cdc`.
+3. Modules that have essentially one correct implementation and are of broad utility in digital designs. This includes :mod:`amaranth.lib.coding`, :mod:`amaranth.lib.fifo`, and :mod:`amaranth.lib.crc`.
+
+The Amaranth standard library is separate from the Amaranth language: everything provided in it could have been implemented in a third-party library.
 
 .. toctree::
    :maxdepth: 2
 
    stdlib/enum
    stdlib/data
    stdlib/wiring
-   stdlib/coding
    stdlib/cdc
-   stdlib/crc
+   stdlib/coding
    stdlib/fifo
+   stdlib/crc