Improve sections in lifetime of documents

In order to make it easier to follow, improve the sections division in the
document about lifetime of documents.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

content/concepts/document_lifetime.md

@@ -84,7 +84,7 @@ Policies            | 24 months             |
 Reference hardware  | 12 months             | Based on Apertis release cadence          |
 Releases            | -                     | Not meant to have an end of life          |
 
-# Process
+# Metadata
 
 Based on the concepts and categories already described the following process
 aims to reduce the gap in-between current status and what the documentation reflects.
@@ -92,9 +92,7 @@ aims to reduce the gap in-between current status and what the documentation refl
 Documents should include metadata which will make it easier to determine the
 moment an update or review is required.
 
-## Metadata
-
-### Dates
+## Dates
 
 Currently each document provides its creation date, which is useful to provide
 information about its age. However, this does not express if the document is
@@ -136,7 +134,7 @@ lastmod = "2021-02-03"
 lastreview =  "2021-05-05"
 ```
 
-### Status
+## Status
 
 Some fields can be used to advertise the status of a document, allowing readers
 to quickly spot important information about a document:
@@ -156,7 +154,7 @@ status = "Deprecated"
 statusDescription = "This document covers frameworks that have been removed in v2022dev2."
 ```
 
-### Tags
+## Tags
 
 Tags are a good way to connect different documents that are related. Based on
 them it is easy to perform a quick search on a specific topic and retrieve the
@@ -202,7 +200,7 @@ document as in the example above:
 tags = ["licensing"]
 ```
 
-## Process
+# Process
 
 This section describes the process that should be put in place to keep track
 of each document's life, from its creation until its end-of-life.