branching: use single "main" branch

[MuellerSeb]

We should simplify our branching structure and use the new default "main" that was suggested by GitHub:

• https://github.blog/changelog/2020-10-01-the-default-branch-for-newly-created-repositories-is-now-main/
• https://github.com/github/renaming
• https://github.blog/changelog/2020-07-17-links-to-deleted-branches-now-redirect-to-the-default-branch/

Things to take care of:

• image links in README.md (hopefully auto-redirect as promised for older docs)
• links to LICENSE
• update CI scripts (only refer to main)
• update coveralls branch list
• update readthedocs standard branch to main

[MuellerSeb]

Done.