docs: specify release version branch

[JPHutchins]

This adds a note to the master branch explaining that different versions of LVGL have corresponding simulator examples. This approach has several deficiencies.

• the README.md now diverges (master is different than the branches)
  • somewhat justifiable in the sense that if you got to a release branch you probably already knew you should be there
• perhaps the REAMDE.md of each branch should clearly state what dependencies they are using; e.g., the master branch would state "LVGL Simulator for latest LVGL master"?
• perhaps this sort of clarification should exist here, https://docs.lvgl.io/latest/en/html/get-started/pc-simulator.html, instead?

[JPHutchins]

A better solution might be for this page, https://docs.lvgl.io/8.2/get-started/pc-simulator.html?highlight=simulator, to link to the compatible 8.2 branch, for example. Though, that's still implicit.

[kisvegabor]

Thank you for the update.

I think most of the people are interested in either the the release/v8.3 or the mater branch. As the master branch's README is visible by default, I think it should act as a hub to navigate people to the right place.

E.g. - as you suggested - say that "it's the master branch for LVGL master, if you need v8.3 click here". I think it's quite simple and still helpful. What do you think?

[JPHutchins]

@kisvegabor Good idea, I will give it a shot!

[kisvegabor]

Looks great, thank you!