doc: make links to GIFs slightly easier to find
doc: make links to GIFs slightly easier to find

My thinking: I was looking through the README from the perspective of a
new user (who will probably just be skimming on a first pass and might
have a mild case of information overload) and realized I made the GIFs a
bit difficult to find in my initial PR
[#2362](https://github.com/lervag/vimtex/pull/2362).
So I've added:
- a few headings specifically for GIFs
- links in the Features section immediately next to the corresponding
  feature, instead of tucked away at the bottom of the list of features.

lervag/vimtex,lervag/vimtex