Update green color to make it darker and add underline

[lisasy]

Proposed design:

The green color #005B30 on white background passes contrast ratio tests for accessibility.

[lisasy]

In the proposed design, the link underline is always visible, but in the deploy preview we can only see it on hover - should we change that as part of this PR too?

I think it looks great, but without the underline maybe it's a bit close to the normal text colour to distinguish the links - what do you think?

Very strange that in the deploy preview, it's only visible on hover :( The intent is for the underline to be visible for the default and hover state.

[lisasy]

Before we merge this let's also remove the algolia info in docusaurus.config.js and replace it with the env variables, unless @Zhao-Andy is okay with keeping them

I see that links are supposed to have the underline, we might want to drop the weight on the links because it looks really heavy - thoughts?

Sounds great — let's drop the weight on the links to use 400 instead of 600.

[aitchiss]

Very strange that in the deploy preview, it's only visible on hover :( The intent is for the underline to be visible for the default and hover state.

I had a wee look and it looks like we have a CSS variable --ifm-link-decoration which is set to none, making the underlines invisible unless hovered. Looks like this is some Docusaurus default that we need to overrided in the :root selector

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