readme: redo the CI badges and reword things a bit

[illwieckz]

Redo the CI badges after:

• Convert CI from Azure Pipelines to Github Actions #2005
• ci: make job names follow our usual convention #2006

The GitHub Action badges should now be per job. I'm not sure we can get a direct link to the job though.
The Appveyor badges are still per-project, they're still valuable because once it's green, each of them is green. I used a diferent URL for the image that references the GitHub instance and not the Appveyor profile, so the image path should survive even if we move Appveyor to another profile (the history link will not).

I believe it's good enough for now. It's already better than before.

Edit: I also reworked and unified some wordings with the game code side:

• readme: redo the CI badges and reword things a bit Unvanquished/Unvanquished#3583

[slipher]

The 32-bit Windows job was nuked at some point so there is only one Appveyor job anyway.

[illwieckz]

Ah yes, thanks.

[illwieckz]

I removed the mistaken windows-i686-msvc badge. Since there was an empty cell in the table I put there a badge for the CodeQL C++ static analysis because why not.

[illwieckz]

I noticed the Web link was mistakenly linking to the forum instead of the blog, also fixed.

[illwieckz]

I also reworked and unified some wordings with the game code side:

• readme: redo the CI badges and reword things a bit Unvanquished/Unvanquished#3583