Documentation: Fix broken URLs

Documentation: Fix broken URLs

- VBT information: The link from 01.org is dead, but appears to have
  been identical to the i915 page in the Linux kernel docs based on
  snapshots on archive.org.
- Cgit: coreboot no longer has cgit running for the repos it hosts,
  and these links redirect to the Gitiles list of repos hosted on
  review.coreboot.org. Based on snapshots on archive.org, these used
  to link to the individual repo or tree. Replace these with an
  equivalent Gitiles link.

Change-Id: Id0bfee7b806c851fbe1dcf357e14d9b593e8569a
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74188
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Patrick Georgi <patrick@coreboot.org>