fix(ext): parse cmdincludemd content as RST so legacy `_include/*.txt` directives render

The `_include/*.txt` library is written in reStructuredText (`.. cfgcmd::`,
`.. code-block::`, `.. note::`, `.. cmdinclude::`). After the RST→MyST
migration, the `{cmdincludemd}` fence on .md pages routed include content
through `MockState.nested_parse`, which in MyST 2.0 simply replays the
content through the MyST renderer (see myst_parser/mocking.py:153). RST
directives in the .txt file then rendered as literal paragraph text:

  <p>.. cmdinclude:: /_include/interface-description.txt</p>
  <p>.. cfgcmd:: set interfaces ethernet ...</p>
  <p>.. code-block:: none</p>

User-visible symptom (reported on rolling docs): the Ethernet, dummy,
tunnel, bonding, bridge, macsec, vxlan, l2tpv3, pseudo-ethernet,
virtual-ethernet, and wireless pages all showed unprocessed `.. cfgcmd::`
and `.. cmdinclude::` directives instead of styled command boxes.

Fix: mirror MyST's own `{eval-rst}` plumbing
(`MockRSTParser().parse(text, doc)` per
`mdit_to_docutils/base.py:1655`). Build a fresh document that inherits
the outer document's settings + reporter, run `MockRSTParser` over the
substituted include content, and graft the resulting children back
into the calling document with explicit-target registration. Keeps the
Sphinx env available to `cfgcmd`/`opcmd`/`cmdinclude` directives.

Verified locally on rolling (Sphinx 7.x, myst-parser 2.0): zero literal
`cfgcmd::` / `cmdinclude::` / `code-block::` / `opcmd::` / `note::` /
`include::` strings remain in any built HTML page under
`configuration/`, `automation/`, `installation/`, `operation/`,
`vpp/`; all 11 affected interface pages now render proper
`cfgcmd-heading` / `cfgcmd-body` blocks (e.g. ethernet.html: 160,
wireless.html: 172, dummy.html: 4).

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