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>

Documentation/acronyms.md

@@ -1037,7 +1037,7 @@ Spec](https://uefi.org/specifications) for details, or run the tool
 * VBIOS - Video BIOS
 * VBNV - Vboot Non-Volatile storage
 * VBT - [**Video BIOS
-  Table**](https://01.org/linuxgraphics/gfx-docs/drm/ch04s02.html#id-1.4.3.4.16)
+  Table**](https://www.kernel.org/doc/html/latest/gpu/i915.html#video-bios-table-vbt)
 * VESA - Video Electronics Standards Association
 * VGA: Video Graphics Array
 * VID: Vendor Identifier

Documentation/index.md

@@ -1,9 +1,8 @@
 # Welcome to the coreboot documentation
 
 This is the developer documentation for [coreboot](https://coreboot.org).
-It is built from Markdown files in the
-[Documentation](https://review.coreboot.org/cgit/coreboot.git/tree/Documentation)
-directory in the source code.
+It is built from Markdown files in the [Documentation] directory in the
+source code.
 
 ## Spelling of coreboot
 
@@ -143,7 +142,7 @@ say hello!
 ## Getting the source code
 
 coreboot is primarily developed in the
-[git](https://review.coreboot.org/cgit/coreboot.git) version control
+[git](https://review.coreboot.org/plugins/gitiles/coreboot) version control
 system, using [Gerrit](https://review.coreboot.org) to manage
 contributions and code review.
 
@@ -200,3 +199,5 @@ Contents:
 * [Acronyms & Definitions](acronyms.md)
 * [External Resources](external_docs.md)
 * [Documentation License](documentation_license.md)
+
+[Documentation]: https://review.coreboot.org/plugins/gitiles/coreboot/+/refs/heads/master/Documentation/