docs: add troubleshooting guide for invalid_client OAuth error
What does this MR do?
Adds a Troubleshooting subsection to the README explaining the invalid_client error that occurs during OAuth login when the OAuth application has the Confidential option enabled.
The error looks like:
Oauth2: "invalid_client" "Client authentication failed due to unknown client, no client authentication included, or unsupported authentication method.".Since glab is a public client and authenticates without a client secret, a Confidential application is rejected by GitLab. The new section documents the cause and the fix (clearing the Confidential checkbox), and links to the existing OAuth setup requirements.
A matching entry was also added to the table of contents.