Update key concepts content pages

[mariaschuld]

Context: The Key Concepts section has been turned into a glossary, but the content is somewhat out of date.

For example:

• Automatic differentiation page: Delete or improve the delineation to symbolic or numeric differentiation
• Circuit ansatz: Differentiate clearly between "template", "ansatz" and "architecture"

[To be continued...]

Solution: We should update and extend the pages one-by-one.

[glassnotes]

Following the completion of #135 and #193, the page of automatic differentiation has been removed from view, and a number of new pages / definitions were added. The pages that were not modified in those two updates are:

• circuit ansatz
• hybrid computation
• parameter-shift
• quantum embedding
• quantum gradient
• quantum neural network
• variational circuit

Do you feel any of those pages would benefit from a refresh? If not, we can close this issue.

[mariaschuld]

Yes, I thnk this issue is outdated and was not very informative to begin with.

[glassnotes]

Cool, thanks for checking! :+1: