Simplify main_nav yml

[m52go]

This PR simplifies the YML markup for _includes/main_nav.html.

Previously, the main_nav markup was broken into parts and included URLs, which was unnecessary and confusing for translators. These changes condense the main_nav markup into a succinct list.

Review Guidance

A thorough visual review of the main navigation menu should suffice to review this pull request. Visit https://deploy-preview-387--bisq-website.netlify.app/ and make sure the main navigation menu at the top matches what's on https://bisq.network for all 9 languages—desktop and mobile. There should be no missing links, etc. Ideally no colliding text either (e.g., from text that's too long).

[Bayernatoor]

Desktop verison checks out except - see screenshots and note below.
Japanese text showing up wrong on proposed branch

Main website:

Mobile tested on Android 8 and IOS 12

Text looks good, navigation menu looks good.

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Desktop and mobile, regardless of language the Getting Started link always redirects the user to the English "Getting Started" page. Perhaps, this hasn't been translated yet?

[m52go]

Japanese text showing up wrong on proposed branch

Thanks, good catch. The translation seems to be correct, but the text is too long for the container. The previous translation was shorter, which is why this wasn't happening before. I've reverted this update with b0bf8e8.

Likelihood of this happening in the future should decrease as a result of Transifex notes indicating maximum length for strings like this.

"Getting Started" page. Perhaps, this hasn't been translated yet?

Yup, that's correct.