Update documentation with missing information on Git-CD mirroring

[Dogacel]

I have struggled on my attempt to set a git-cd mirroring from GitHub with a fine-grained access token. So wanted to document some of the things,

On, page for mirroring
https://line.github.io/centraldogma/mirroring.html#setting-up-a-git-to-cd-mirror

1. accessToken is only supported for git+https schema.
2. localRepo needs to be created manually otherwise it won't log any message indicating it doesn't exist.
3. Getting started section points to docker hub so I accidentally downloaded an old image and it did not support accessToken.

[minwoox]

Yeah, we certainly need to update it. 😓

[thachlp]

Hi @Dogacel @minwoox, when setting up git-cd mirroring from GitHub, I also face the problem when missing the localRepo, so I think we should add a WARNING log for this.
Please help review it 🙇