title updates for docs

docs/sources/installation/_index.md

@@ -13,6 +13,8 @@ labels:
 weight: 200
 ---
 
+# Installation
+
 ## Choosing plugin version
 
 Currently (in version `4.x.x`) Grafana-Zabbix plugin supports Zabbix versions `4.x` and `5.x`. Zabbix `3.x` is not supported anymore. Generally, latest plugin should work with latest Grafana version, but if you have any compatibility issue, try to downgrade to previous minor release of Grafana. It's also helpful to report (but use search first to avoid duplicating issues) compatibility issues to the [GitHub](https://github.com/grafana/grafana-zabbix/issues).

docs/sources/installation/building-from-sources.md

@@ -13,6 +13,8 @@ labels:
 weight: 210
 ---
 
+# Building from sources
+
 If you want to build a package yourself, or contribute - here is a guide for how to do that.
 
 ## Prerequisites

docs/sources/installation/upgrade.md

@@ -13,6 +13,8 @@ labels:
 weight: 220
 ---
 
+# Upgrade
+
 ## Upgrade from 2.x
 
 After [enabling](../../configuration/#enable-plugin) Zabbix App go to _Data Sources_, open your configured Zabbix