Decrease the font size and the padding of the `.anchor` icons used in the README

_Originally opened at !2819 by @roperzh._

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## What does this MR do?

Removes an overlap between the anchor icons on the readme and the sidebar on small screens by reducing the size and the padding of the icon.

## Are there points in the code the reviewer needs to double check?

Probably someone in the Design / UX team will need to review if the solution makes sense.

## What are the relevant issue numbers?

Issue #13334

## Screenshots

![Screen_Shot_2016-02-15_at_3.35.13_PM](/uploads/0e8b215809eee8efec9b6a136b0bd841/Screen_Shot_2016-02-15_at_3.35.13_PM.png)

See merge request !3150