Documentation: Fix sphinx warnings

This fixes the following warnings:

mainboard/starlabs/common/flashing.md::
WARNING: image file not readable:
- mainboard/starlabs/common/fwupdVersion.png
- mainboard/starlabs/common/BiosLock.jpg
- mainboard/starlabs/common/SwitchBranch.png

cbfstool/index.md::
WARNING: document isn't included in any toctree

internals/devicetree_keywords.md::
WARNING: document isn't included in any toctree

mainboard/asus/wifigo_v1.md::
WARNING: document isn't included in any toctree

mainboard/google/index.md::
WARNING: document isn't included in any toctree

mainboard/starlabs/common/flashing.md::
WARNING: document isn't included in any toctree

releases/boards_supported_on_branches.md::
WARNING: document isn't included in any toctree
WARNING: None:any reference target not found:
- releases/coreboot-4.16-relnotes
- releases/coreboot-4.15-relnotes
- releases/coreboot-4.14-relnotes
- releases/coreboot-4.13-relnotes
- releases/coreboot-4.12-relnotes
- releases/coreboot-4.11-relnotes
- releases/coreboot-4.10-relnotes
- releases/coreboot-4.9-relnotes
- releases/coreboot-4.8.1-relnotes
- releases/coreboot-4.7-relnotes
- releases/coreboot-4.6-relnotes
- releases/coreboot-4.5-relnotes
- releases/coreboot-4.4-relnotes
- releases/coreboot-4.3-relnotes
- releases/coreboot-4.2-relnotes
- releases/coreboot-4.1-relnotes
- ../../src/soc/intel/common/block/cse/cse.c

Change-Id: I22273bc1bc34b6297cef4e594c454c2316d4215a
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64576
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>

Documentation/mainboard/starlabs/common/flashing.md

@@ -16,7 +16,7 @@ fwupdmgr --version
 ```
 
 This will show the version number. **1.5.6** or greater will work.
-![fwupd version](fwupdVersion.png)
+![fwupd version](../fwupdVersion.png)
 On Ubuntu 20.04, Ubuntu 20.10, Linux Mint 20.1 and elementaryOS 6, fwupd 1.5.6 can be installed from our PPA with the below terminal commands:
 
 ```
@@ -40,7 +40,7 @@ BIOS Lock must be disabled when switching from the standard AMI (American Megatr
 2\. When the BIOS settings load, use the arrow keys to navigate to the **Advanced** tab\. Here you will see **BIOS Lock**\.
 3\. Press `Enter` to change this setting from **Enabled** to **Disabled**
 
-![Disable BIOS Lock](BiosLock.jpg)
+![Disable BIOS Lock](../BiosLock.jpg)
 
 4\. Next, press the `F10` key to **Save & Exit** and then `Enter` to confirm.
 
@@ -61,7 +61,7 @@ fwupdmgr switch-branch
 ```
 
 You can then select which branch you would like to use, by typing in the corresponding number:
-![Switch Branch](SwitchBranch.png)
+![Switch Branch](../SwitchBranch.png)
 You will be prompted to confirm, press `y` to continue or `n` to cancel.
 
 Once the switch has been completed, you will be prompted to restart.