Added additional support for git documentation sources

[sei-mpriest]

This PR adds some improvements for the handling of mkdocs materials when using git as a source for the docs. The changes are summarized as follows:

1. Allows the use of the existing persistent volume claim for both the cronjob and the init containers of the deployment when running using git
2. Supports overriding the site_url of the mkdocs.yml file when the deployment is served at a different host than the git repo specifies in the site_url field
3. Supports overriding the path that nginx uses to redirect traffic to the static website to match the value specified in site_dir rather than the default value of "site"
4. Supports providing git credentials as a secret in the format used by the git store credential helper

Note:
Because this PR configures git to use the store git credential helper, but the config file is read-only, we see this error in the init container logs every time git tries to write the creds:

fatal: unable to get credential storage lock in 1000 ms: Read-only file system

This error does not impact the functionality because the credentials did not actually change and the read of the credentials works properly. This could be fixed by using a custom git credential helper, but that would require some additional development. Is it ok to leave the current implementation given that error in the logs or should we fix it?

[sei-mkaar]

@sei-mpriest Nice! 👍

And I agree about adding git https auth to this PR.

[sei-mpriest]

Marked as draft pending the git creds change.