fix(theme): link hover color inside a custom block

[skirtles-code]

This PR explicitly sets the a:hover style for links inside custom blocks.

A link inside a warning block uses the yellow theme, but the :hover uses the brand theme:

There's a similar problem for danger blocks.

It's a bit more complicated for tip, info and details blocks. In theory, they have the same problem. However, the current CSS overrides for those 3 custom blocks set the link color to --vp-c-brand-1, which is what it would be anyway. As a result, there isn't a visual inconsistency on :hover, as it just inherits the default color of --vp-c-brand-2. I've chosen to include an explicit :hover override for those 3 custom blocks too. It seems inconsistent to set the color for a but not a:hover.