readme: add CI badges

[illwieckz]

Add CI badges now that we have working CI.

[illwieckz]

The badge are not seen from GitHub but seen when I'm logged in into Azure. I noticed the Azure projet is set to “private” instead of “public”, I don't know how to change that. That may also explain why the CI is so slow because it does only one CI job at a time unlike our public Azure projects.

[illwieckz]

OK, I could enable the anonymous badges at organization level to be able to enable them at the project level.

It would be better if the project was public.

[slipher]

I guess we don't want these in view of Unvanquished/Unvanquished#3577.

[illwieckz]

I now ported the CI badges to the ported GitHub Actions CI.

Also fixed some formatting, the platform status wasn't printed one platform per line, I could have add line breaking two-spaces ending but went the list way. I also unified the whitespacing (two empty lines before a title, like the convention used in our other READMEs and already partially implemented there).

[illwieckz]

To be fully tested one should wait for #73 to be merged to make sure badges are available and generated:

• Convert CI from Azure Pipelines to Github Actions #73

[illwieckz]

So there was a small mistake in the badge URLs, they're now working.

[illwieckz]

As a side note I notice it runs the CI even when the code isn't modified.