Add mjs_makeFlex to the MuJoCo C and Python APIs.

- Add mjs_makeFlex C API that wraps mjCFlexcomp::Make(), creating
  a flex with auto-generated bodies, joints, and equality constraints
  from parameters (type, dim, dof, count, cellcount, spacing, scale,
  radius, mass, inertiabox, equality, rigid, flatskin, elastic2d,
  pos, quat, origin, file, vfs).
- Add Python binding body.make_flex() in specs.cc with keyword args.
- Refactor flex_spec_provision.py to use make_flex() instead of XML
  string templating + separate spec + attach. Eliminates textwrap
  dependency.
- Merge user_flexcomp into user_model BUILD target to resolve
  circular dependency.

PiperOrigin-RevId: 922725304
Change-Id: Ice13787c45f3173407fec659f9590a6453bc65dd

doc/APIreference/functions.rst

@@ -4496,6 +4496,17 @@ Add sensor.
 
 Add flex.
 
+.. _mjs_makeFlex:
+
+`mjs_makeFlex <#mjs_makeFlex>`__
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_makeFlex
+
+Add flexcomp: create flex with auto-generated bodies/joints, return flex spec.
+
+*Nullable:* ``type``, ``dof``, ``count``, ``cellcount``, ``spacing``, ``scale``, ``pos``, ``quat``, ``origin``, ``file``, ``vfs``
+
 .. _mjs_addPair:
 
 `mjs_addPair <#mjs_addPair>`__