[help] convert search_syntax.rst to help page

searx/help/en/search-syntax.md

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+# Search syntax
+
+SearXNG allows you to modify the default categories, engines and search language
+via the search query.
+
+Prefix `!` to set Category/engine.
+
+Prefix: `:` to set the language.
+
+Abbrevations of the engines and languages are also accepted.  Engine/category
+modifiers are chainable and inclusive (e.g. with [!it !ddg !wp
+qwer]($search?q=!it+!ddg+!wp+qwer)) search in IT category **and** duckduckgo
+**and** wikipedia for ``qwer``).
+
+See the [preferences]($preferences) for the list of engines,
+categories and languages.
+
+## Examples
+
+Search in wikipedia for `qwer`:
+
+* [!wp qwer]($search?q=!wp+qwer)
+* [!wikipedia qwer]($search?q=!wikipedia+qwer)
+
+Image search:
+
+* [!images Cthulu]($search?q=!images+Cthulu)
+
+Custom language in wikipedia:
+
+* [:hu !wp hackerspace]($search?q=:hu+!wp+hackerspace)