Replace master with main in the docs

[dstansby]

It is now common to use main as the core branch in repositories (see https://sfconservancy.org/news/2020/jun/23/gitbranchname/). This PR updates the docs to use main instead of master where possible. It would be great to also:

• Rename the master branch to main in the asv repository
• Change the default branch in the code similarly

but I think those are bigger changes beyond my ability.

[datapythonista]

Thanks @dstansby, good idea. I guess there are probably more repos in the ecosystem using main now.

Do you mind merging master into your PR please. We just added the docs to the CI. Everything looks good, but wanted to have a docs build in the CI before merging, just to make sure.

[datapythonista]

Thanks @dstansby