chore: Add CI badges to README.md

[ChinoUkaegbu]

Adds badges for main branch tests and weekly tests to README

Fixes #1984

• [ ] Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• [ ] Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged.)

[dstebila]

Can be merged after the test suite passes. (I thought we might be able to merge without running CI since it only affects documentation, but I remembered that we sometimes have Markdown weirdness between Doxygen and Github Markdown, so running the CI to make sure that passes.)