doc: add extra_styles/config template notes

[Lx]

• State that extra_styles are not merged in config template usage.
• Suggest a workaround.

[afarago]

I am using another workaround:

in button_card_templates:

variables:
    var_extra_styles: |-
      ...
  extra_styles: |-
    [[[ return variables.var_extra_styles; ]]]

in the actual button:

    extra_styles: |-
      [[[
        return variables.var_extra_styles + " ... "