Update Geo docs: Git pull/push over SSH against a secondary site

Problem to solve

Update the docs to reflect the changes in &11026.

Proposal

Update at least the following docs:

• doc/development/geo/proxying.md
• ??

Here is the new Git pull over SSH flow when the secondary detects (during the authorized_keys internal API call) that its copy of the repo may not be up-to-date:

sequenceDiagram
    participant C as Git on client
    participant S as GitLab Shell
    participant I as Workhorse & Rails
    participant P as Workhorse & Rails

    Note left of C: git pull/clone
    Note over S,I: Secondary site
    Note over P: Primary site
    C->>+S: ssh git upload-pack request
    S->>I: SSH key validation (api/v4/internal/authorized_keys?key=..)
    I-->>S: HTTP/1.1 300 (custom action status) with {endpoint, msg, primary_repo, authorization headers}
    S->>P: POST $PRIMARY/foo/bar.git/info/refs/?service=git-upload-pack
    P-->>S: HTTP/1.1 200 OK
    P-->>S: <response>
    S-->>C: return Git response from primary
    C-->>S: stream Git data to push
    S->>P: POST $PRIMARY/foo/bar.git/git-upload-pack
    P-->>S: HTTP/1.1 200 OK
    P-->>S: <response>
    S-->>-C: gitlab-shell upload-pack response

Here is the new Git push over SSH flow at a secondary:

sequenceDiagram
    participant C as Git on client
    participant S as GitLab Shell
    participant I as Workhorse & Rails
    participant P as Workhorse & Rails

    Note left of C: git fetch
    Note over S,I: Secondary site
    Note over P: Primary site
    C->>+S: ssh git receive-pack request
    S->>I: SSH key validation (api/v4/internal/authorized_keys?key=..)
    I-->>S: HTTP/1.1 300 (custom action status) with {endpoint, msg, primary_repo}
    S->>I: POST /-/push_from_secondary/2/foo/bar.git/info/refs/?service=git-receive-pack
    I->>P: Proxy to primary
    P-->>I: HTTP/1.1 200 OK
    I-->>S: <response>
    S-->>C: return Git response from primary
    C-->>S: stream Git data to push
    S->>I: POST /-/push_from_secondary/2/foo/bar.git/git-receive-pack
    I->>P: Proxy to primary
    P-->>I: HTTP/1.1 200 OK
    I-->>S: <response>
    S-->>-C: gitlab-shell receive-pack response

Who can address the issue

groupgeo

Other links/references