docs: add Lucene query syntax documentation for Search Form

[Manuel-dev-ing]

This PR adds documentation for Lucene query syntax usage in the Search Form.

Fixes #7156

[agriffard]

Add an item below

• Guides:
  in mkdocs.yml

  OrchardCore/mkdocs.yml

  Line 157 in 6025ca4

  - Manage Assets and NPM dependencies:

  so a link appear under the Guides menu.

[Manuel-dev-ing]

Done, I've added it to the Guides section in mkdocs.yml.

[agriffard]

Ok

[github-actions]

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