Add docs

Author: gaogaotiantian

Doc/library/pdb.rst

@@ -699,6 +699,13 @@ can be overridden by the local file.
       :pdbcmd:`interact` directs its output to the debugger's
       output channel rather than :data:`sys.stderr`.
 
+.. pdbcommand:: attach process
+   
+   Attach to a running process.  The *process* argument could be either a
+   :class:`subprocess.Popen`, :class:`multiprocessing.Process` or a process ID.
+
+   .. versionadded:: 3.15
+
 .. _debugger-aliases:
 
 .. pdbcommand:: alias [name [command]]

Doc/whatsnew/3.15.rst

@@ -89,6 +89,12 @@ New modules
 Improved modules
 ================
 
+pdb
+---
+
+* :pdbcommand:`attach` command has been added to attach to a running process.
+  (Contributed by Tian Gao in :gh:`133954`.)
+
 ssl
 ---