docs: add documentation on fixed content

[brandyscarney]

ionic-team/ionic-framework#5987

[movrack]

The feature always exist? I can't find it.

[brandyscarney]

@movrack Yes, ion-fixed was converted into an attribute. See the following test for an example of it being used: https://github.com/ionic-team/ionic/blob/master/src/components/nav/test/basic/pages/primary-header-page/primary-header-page.ts#L31

And a screenshot of that test (the fixed button and large grey box are fixed):

[ix-xerri]

It partly worked. The content stays in .scroll-content and fixed content remains empty and I am using ion-segment and the segment where I don't have google map but an ion-card does not show. The content is hidden.

[Shehab-Muhammad]

I can't use ion-fixed right now, is it possible to put content into container with class .fixed-content ? how ?