Changing default branch to `develop`

[Yauheni Prakopchyk]

We want to change default branch to develop to keep development approach in sync with vuestic-ui. (there we have develop branch as main, and master is just a pointer to docs).

Why?

• prs would automatically select develop as destination. Right now it's master and for all prs we have to be careful.

Why not do it right now?

• The problem is that installation instructions for vuestic admin use default branch (git clone uses default branch, I checked) and I would prefer not to complicate instructions too much. I.e. user would have to clone, then checkout.

So maybe we need a proper installer.