Documentation: Fix header levels

This fixes the following MyST Parser warnings:

- Non-consecutive header level increase
- Document headings start at H2, not H1

The header levels (the number of "#" characters before a heading) are
intended to form a logical hierarchy of each section and subsection in a
document. A subsection typically should have a header level one more
than its parent section. Most of these warnings are caused by extra "#"
characters, which were simply removed, or sections missing a "#"
character to make it fall under its parent section.

Notable changes:

getting_started/kconfig.md: Changed the header level of the "Keywords"
section from 2 to 3 to fall under "Kconfig Language" (level 2), and
increased the level of each keyword from 3 to 4 to remain under
"Keywords". This also fixes the warnings of "H3 to H5" increases, since
the Usage/Example/Notes/Restrictions sections for each keyword had a
level of 5.

soc/intel/cse_fw_update/cse_fw_update.md: Changed the first line to a
top level header acting as the title of the document. Without this
soc/intel/index.md displays all the level 2 headers in this document
instead of a single link to cse_fw_update.md.

Change-Id: Ia1f8b52e39b7b6524bef89a95365541235b5b1b9
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83382
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

Documentation/mainboard/starlabs/common/flashing.md

@@ -1,6 +1,6 @@
 # Flashing with fwupd
 
-#### **Requirements:**
+## **Requirements:**
 
 * fwupd version 1.5.6 or later
 * The battery must be charged to at least 30%
@@ -44,7 +44,7 @@ BIOS Lock must be disabled when switching from the standard AMI (American Megatr
 
 4\. Next, press the `F10` key to **Save & Exit** and then `Enter` to confirm.
 
-#### **Switching Branch**
+## **Switching Branch**
 
 Switching branch refers to changing from AMI firmware to coreboot, or vice versa.