[docs] move blog article "Offline engines" to dev/offline_engines.rst

The article "Offline engines" should be in developer's documentation next to
chapter "Engine overview".

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/blog/index.rst

@@ -7,6 +7,5 @@ Blog
    :caption: Contents
 
    lxcdev-202006
-   intro-offline
    private-engines
    search-indexer-engines

docs/dev/index.rst

@@ -9,6 +9,7 @@ Developer documentation
    quickstart
    contribution_guide
    engine_overview
+   offline_engines
    search_api
    plugins
    translation