Clarify intro to Personal access tokens

What does this MR do?

As noted in #36492 (closed), our doc on how to use personal access tokens with our API are not clear.

My intent with this MR is to clarify the intro, by explicitly pointing to https://docs.gitlab.com/ee/api/README.html#personal-access-tokens as an example of how readers can use a personal access token.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Style guides
• Separation of EE specific content

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team