Docs: Address remaining documents not included in toctrees

Sphinx outputs "document isn't included in any toctree" warnings for a
few files in the Documentation tree, so address this by adding them to
toctrees or explicitly marking them as excluded.

- mb/starlabs/common/building.md: Add to the Star Labs toctree in
  mainboard/index.md
- RFC/intel-gpio-cleanup.md: Mark as orphan to explicitly exclude it
  from the docs.
- drivers/dt_entries.md: This was already accessible through an inline
  link in drivers/index.md, but links do not add items to toctrees. Add
  a hidden toctree listing dt_entries.md to define its heirarchy in the
  documentation while preserving the inline link instead of moving the
  link to a single item list like a normal toctree would. The content of
  this document did not fit the existing toctree in drivers/index.md,
  which appears to list drivers, while dt_entries discusses connecting
  those drivers to the devicetree.

Change-Id: I5fd6851a3adf6c91d81298fc61f773dae6eeca19
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84245
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

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