Fix whitespace in many administration docs

Many code blocks are 4spaced, and they render in GitLab
without coloring as a result, even though they are
fenced with a language label. If in a list, other items
will render as being in a code block too, even if not
meant to. This fixes all these issues for many admin
docs (part 2)

doc/administration/integration/plantuml.md

@@ -74,6 +74,7 @@ our AsciiDoc snippets, wikis and repos using delimited blocks:
   Alice -> Bob : Go Away
   ```</pre>
 
+
 - **AsciiDoc**
 
    ```

doc/administration/operations/filesystem_benchmarking.md

@@ -81,16 +81,19 @@ executed, and then read the same 1,000 files.
    ```sh
    mkdir test; cd test
    ```
+
 1. Run the command:
 
    ```sh
    time for i in {0..1000}; do echo 'test' > "test${i}.txt"; done
    ```
+
 1. To benchmark read performance, run the command:
 
    ```sh
    time for i in {0..1000}; do cat "test${i}.txt" > /dev/null; done
    ```
+
 1. Remove the test files:
 
   ```sh

doc/administration/pages/index.md

@@ -395,6 +395,7 @@ Follow the steps below to configure GitLab Pages in a separate server.
    node_exporter['enable'] = false
    gitlab_rails['auto_migrate'] = false
    ```
+
 1. Run `sudo gitlab-ctl reconfigure`.
 1. On `app1` apply the following changes to `/etc/gitlab/gitlab.rb`: