README correction to docs path & additional gitignore to macos + vscode

[planetf1]

1. Add common macOS filetype to .gitignore to avoid accidental checkins
2. Add visual studio code / jetbrains IDE config file location to .gitignore as above
3. Correction to readme - path to generated documentation was incorrect

Both are very minor, but things I noticed when using liboqs for the first time, and walking through the documentation.
Hope this really small contribution helps & is appropriate.

I can also squash or re-edit to follow appropriate guidelines if needed

• 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.) -> NO
• 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 oqs-provider, OQS-OpenSSL, OQS-BoringSSL, and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.) -> NO

No tests executed - this is just a Readme & .gitignore update

[baentsch]

LGTM. Thanks for the contribution!