DOC-580 | Improve Docs styling and add ArangoGraph Banner

[nikhil-varma]

Description

• This PR improves the overall styling of the docs and matches the ArangoGraph's UI styles.
• Also adds ArangoGraph's banner to the home page of each Doc version specifically 3.10, 3.11 & 3.12

[arangodb-docs-automation]

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https://deploy-preview-244--docs-hugo.netlify.app

[Simran-B]

The text contrast of inline code is not good, and way too low if it's in a link. The link underline is also not visible below inline code (see "db object" link):

It doesn't look good and has too little contrast in headlines as well:

The copy button overlapping the content is not nice. There would be space to make the code box wider in this case, but to improve it more generally, could we show the copy button only on hover over the code box?

Boxes like here need to be restyled still. Having the same border radius on nested boxes looks wrong:

[nikhil-varma]

@Simran-B
Links & contrast and the copy button overlap issues are fixed.

Any further changes should be a part of a separate ticket. It becomes tricky to track changes. Let's keep the scope as-is for this one. Please use this ticket to write down whatever is necessary for the next set of issues: https://arangodb.atlassian.net/browse/DOC-585

[palashkaria]

LGTM!