Add back next and previous buttons to footer

[javabster]

In an earlier version of the theme we removed the "Next" and "Previous" buttons in the footer. In response to user requests and further discussion we have decided to add that component back.

fixes #174

In this PR:

• added back the original code for the next/previous buttons
• added a small line break at the very end of the footer so that the copyright text doesn't sit so close to the edge of the page.

[HuangJunye]

Looking at the old footer.html, it has theme_prev_next_buttons_location environment variable to specify the location. breadcrumb.html still has such logic to show on the top. Do you want add the condtion back?

[HuangJunye]

A quick check, the meta doc and nature doc sets `'prev_next_buttons_location': 'bottom'

• https://github.com/Qiskit/qiskit/blob/master/docs/conf.py#L243
• https://github.com/Qiskit/qiskit-nature/blob/main/docs/conf.py#L212

So practically remove the conditions won't change anything, but I think we can simply revert it back to the old version and let individual repo to decide where to put the button.

[javabster]

I can't see that variable being used anywhere from the qiskit repo side, i think it's used to specify where on the page the buttons are located so I don't think we need them as (I assume) we always want the buttons to be placed at the bottom of the page.

Also I have an open issue for cleaning up the breadcrumbs file, it was one of the few I didn't get round to cleaning up before the last big release we did

[HuangJunye]

I can't see that variable being used anywhere from the qiskit repo side]

It's used at least for qiskit meta and qiskit-nature, probably other repos too.

• https://github.com/Qiskit/qiskit/blob/master/docs/conf.py#L243
• https://github.com/Qiskit/qiskit-nature/blob/main/docs/conf.py#L212

[HuangJunye]

I tested building using prev_next_buttons_location = 'top' release 1.8.7 which is before the the layout refactor #119 and the buttons are broken. So if we want to add that option, we need to fix breadcrumb.html as well.

Source: html.zip

And a quick search on the qiskit github org, all repos that uses prev_next_buttons_location only sets it to bottom so I agree with that we can go ahead and remove the conditions.