Add new page for different login providers

[gounux]

This PR adds a new page to the doc, regarding different ways of login, using social / third-party providers (SSO).

Here are screenshots of the mermaid sequence diagrams :

• Web:

• QField / QFieldSync:

[lukasgraf]

@gounux nice work!
As discussed, I have pushed a commit with some additions of mine. I added a bit of introductory text, and extended the first diagram. It now looks like this:

The second flow / diagram is a bit more difficult. I'll see what I can come up with for that one.

[suricactus]

@lukasgraf @gounux can we move this forward?

[lukasgraf]

@suricactus @gounux I have extended the description of the OIDC flow for QField / QFieldSync.
Diagrams should be rendered properly in the rich diff here on GitHub.
From my point of view, this could be merged like this.

[gounux]

Thanks a lot @lukasgraf !

PR description updated.

[suricactus]

I mostly checked for readability and spelling issues. Looks good to me.