fix(includes): break the RST `.. include::` chain that pulls need_improvement.txt on sagitta

`_include/interface-vlan-8021ad.txt` started with
`.. include:: /_include/need_improvement.txt`. After PR #2008 the
`{cmdincludemd}` directive parses included `.txt` content as RST, so
the three MD pages that consume interface-vlan-8021ad.txt
(ethernet.md:161, virtual-ethernet.md:46, wireless.md:485) reach
need_improvement.txt through an RST parser.

`need_improvement.txt` is now MyST (`::::{only} not latex / :::{admonition}`),
so the RST parser would render those directives as literal text on
three interface pages.

Rolling does not have this regression — rolling's
`_include/interface-vlan-8021ad.txt` is MyST/MD content and does not
include need_improvement.txt at all. The sagitta file is older
RST-shape content with the include chain at the top, an artifact of the
mid-migration state on sagitta.

Strip line 1 (`.. include:: /_include/need_improvement.txt`) and the
blank line that follows. The QinQ overview content below is untouched
and still parses as RST under `{cmdincludemd}`.

Spotted by Copilot inline review on #2026.

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