[TASK] Index Changelog files as a single snippets

[soee]

With this commit, all Changelog files will be treated as a single snippet when importing their content into the search index.

The underlying logic to parse documentation file content remains unchanged:

• Files excluded by configuration or dedicated meta tags will still be excluded
• All snippets from a file will be fetched

Next, for Changelog files, the snippets from a single file are merged into a single snippet:

• The title (headline) from the first snippet will be used as the title for the merged snippet
• The merged content is created by concatenating the content of the first snippet with the title and content of every subsequent snippet.

Important: After merging, the search index must be cleared and the indexing process must be re-run.

Resolves #29