Update repo to use new qiskit sphinx theme

[javabster]

Summary

The latest version of the qiskit_sphinx_theme (1.10.0) includes 2 major new features:

• It replaces the existing top menu with the same top menu from qiskit.org
• It adds support for segment analytics (optional)

This PR removes the existing docs/templates/layout.html so that terra will stop overriding the qiskit_sphinx_theme and enable the new top menu to show. This PR also updates the conf.py to opt-in to analytics (details on what is tracked here)

I also updated the headings in the left sidebar, because some pages (e.g. tutorials) are specific to terra and not present in the new unified top bar. Similarly other pages (like contributing things) are relevant to all repos and are linked in the new top menu so don't need to be in the sidebar anymore.

Not sure if I did the env var setting the way you meant @mtreinish 😅

Details and comments

[javabster]

my suggested new sidebar layout, based on the diataxis framework we are trying to follow going forward.

[javabster]

thanks for the comments @jakelishman @HuangJunye ! I think i've addressed them all, lmk if theres anything else 😄

[HuangJunye]

I am not tagging automerge in case @jakelishman and others want to have a final look.