docs/auth: clarify OAuth and access token setup

[eugenio-sanchez-sg]

What changed

Updated the README to clearly explain how to connect and authenticate to a Sourcegraph instance.

Added

• Instructions for setting SRC_ENDPOINT on macOS, Linux, and Windows.
• A note that src defaults to https://sourcegraph.com when SRC_ENDPOINT is not set.
• A dedicated OAuth section explaining:
• Run src login to authenticate interactively.
• OAuth does not require SRC_ACCESS_TOKEN.
• OAuth credentials are stored in the operating system’s native keyring.
• SRC_ENDPOINT must remain set so future commands can find the correct OAuth credential.
• Passing a URL to src login only selects the endpoint for that login command.
• Documentation for:
• src auth token, which prints the active token.
• src auth token --header, which prints a complete Authorization header.
• A dedicated personal access token section explaining:
• Access tokens are recommended for CI, scripts, and other non-interactive environments.
• Both SRC_ENDPOINT and SRC_ACCESS_TOKEN must be set.
• SRC_ACCESS_TOKEN takes precedence over an OAuth credential.
• Running src login is unnecessary when SRC_ACCESS_TOKEN is set.
• Shell and PowerShell examples for both endpoint and access-token configuration.
• src auth to the README’s list of available commands.
• The previous section anchor so existing links to the old heading continue to work.

Removed

• The outdated introduction that instructed users to run src login SOURCEGRAPH-URL.
• The implication that passing a URL to src login configures future commands.
• The old combined instructions that treated SRC_ENDPOINT and SRC_ACCESS_TOKEN as required together for every login.
• The duplicated macOS, Linux, and Windows configuration sections.

This is a documentation-only change.