docs: make sure there's only one # markdown header in each file

@bertob wants us to be strict here, and only have one "#" header per
markdown file, and use "##" (or "###", …) for all others. Interestingly,
we mostly got this right already, but this fixes a few cases where this
wasn't correct.

docs/AUTOMATIC_BOOT_ASSESSMENT.md

@@ -56,7 +56,7 @@ components:
   script can optionally create boot loader entries that carry an initial boot
   counter (the initial counter is configurable in `/etc/kernel/tries`).
 
-# Details
+## Details
 
 The boot counting data `systemd-boot` and `systemd-bless-boot.service`
 manage is stored in the name of the boot loader entries. If a boot loader entry
@@ -149,7 +149,7 @@ scenario the first 4 steps are the same as above:
 12. On the following boot (and all subsequent boots after that) the entry is
     now seen with boot counting turned off, no further renaming takes place.
 
-# How to adapt this scheme to other setups
+## How to adapt this scheme to other setups
 
 Of the stack described above many components may be replaced or augmented. Here
 are a couple of recommendations.
@@ -180,7 +180,7 @@ are a couple of recommendations.
    wrap them in a unit and order them after `boot-complete.target`, pulling it
    in.
 
-# FAQ
+## FAQ
 
 1. *Why do you use file renames to store the counter? Why not a regular file?*
    — Mainly two reasons: it's relatively likely that renames can be implemented

docs/CODING_STYLE.md

@@ -543,7 +543,7 @@ layout: default
   time you need that please immediately undefine `basename()`, and add a
   comment about it, so that no code ever ends up using the POSIX version!
 
-# Committing to git
+## Committing to git
 
 - Commit message subject lines should be prefixed with an appropriate component
   name of some kind. For example "journal: ", "nspawn: " and so on.

docs/TRANSLATORS.md

@@ -69,7 +69,7 @@ Once you're done, create a git commit for the update of the `po/*.po` file you
 touched. Remember to undo the changes to the other `*.po` files (for instance,
 using `git checkout -- po/` after you commit the changes you do want to keep.)
 
-# Recompiling Translations
+## Recompiling Translations
 
 You can recompile the `*.po` files using the following command: