Add explanation of manifest and its context in Updatecli (#2059)

* Add explanation of manifest and its context in Updatecli

Added a detailed explanation about what a manifest is and its role in Updatecli

* Update content/en/docs/prologue/introduction.md

---------

Co-authored-by: Olivier Vernin <olivier@vernin.me>

Author: yashpal2104

content/en/docs/prologue/introduction.md

@@ -20,6 +20,8 @@ It reads a manifest then works into three stages:
 2. **Target**, which describes what to update using information defined by the _source_.
 3. **Condition**, which defines a condition, required to be satisfied, to update the target.
 
+*Manifest* - A manifest is a configuration file that provides metadata about other files or the configuration of a system. In the context of Updatecli, a manifest specifies the rules and parameters for automating updates to dependencies, versions, and other configuration files within a repository.
+
 ## Why?
 
 Deciding **how**, **when**, **where** to update information is hard.