Improve content headings

[DavidOliver]

simplescreenrecorder-2024-11-26_15.04.06.mp4

Important

@josevalim, if you'd prefer the hover links to remain invisible until the heading is hovered/focussed, let me know and I'll revert that change.

Allows selection of heading text. Also makes hover link always (slightly) visible for discoverability.

Also improves:

• hover link colour
• hover link icon spacing
• heading spacing

Fixes #1969

[DavidOliver]

I forgot to check the cheatsheet headings, which are also affected by making the hover link visible with a low opacity.

They don't show when previewing printing, but maybe this is a bit much? Should I revert for the cheatsheets, or even entirely?

[josevalim]

Please let's keep it visible only on hover please.

And if we are going to change the color, can you please make sure it works with mix docs --proglang erlang? Generally speaking, I prefer to avoid accent, exactly because we need to make sure it works for both Erlang and Elixir, both on light and dark mode. So it is a bit too much. :)

[DavidOliver]

👍 Done.

The accent colours are handled by the --main... custom properties, and tested in light and dark modes.

[josevalim]

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