Add documentation on charmhub

### Problem statement

The charm does not currently have any documentation present on charmhub for new perspective users of the charm.

### Enhancement Proposal

Add minimal, yet comprehensive documentation about git-integrator usage. This can be done by adding the docs to charmhub discourse and linking the documentation with the charm's listing.

### What needs to get done?

The following documentation must be added to the charm's landing page:
1. Tutorial
- A tutorial displaying how to deploy and configure the charm
2. How-to
- How to configure credentials (username + personal access token) for a remote git repo
- How to configure SSH keys (ssh private key) for a remote git repo
3. Reference
- Information about strict-host-key-checking + the need for consumers containing proper CA chains if it is enabled