User guide: document how to make the topNav sticky

[damithc]

i.e., <header class="header-fixed">

[damithc]

@le0tan is this feature stable enough to go public? I remember there were some quirks to be fixed first

[ang-zeyu]

documented (<header fixed>) and fixed in #1191. It was added to the user guide as well, I haven't seen any issues so far

[damithc]

Oh, nice. Thanks both.

[le0tan]

@damithc I added the feature to the documentation to indicate that it’s ready for use. Ping me if you experienced any issue related to it. Thanks!

[damithc]

@damithc I added the feature to the documentation to indicate that it’s ready for use. Ping me if you experienced any issue related to it. Thanks!

Got it. Thanks @le0tan
Need your help with #1252 though