docs(lv_conf): show how to include something in lv_conf.h

[kisvegabor]

Description of the feature or fix

Fixes #5707

Notes

• Update the Documentation if needed.
• Add Examples if relevant.
• Add Tests if applicable.
• If you added new options to lv_conf_template.h run lv_conf_internal_gen.py and update Kconfig.
• Run scripts/code-format.py (astyle version v3.4.10 needs to be installed) and follow the Code Conventions.
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