docs(auth): add link to FAQ on invalid token error

[agilgur5]

Motivation

• link was requested by Alex, fixes link to SSO docs when login fails: token not valid for running mode #5832

• also shorten the error message to "token not valid" as, in my experience at least, "for running mode" actually significantly increased confusion (c.f. token not valid for running mode after SSO authentication #4991 (comment))

  • everyone on my teams thought that it meant we misconfigured our auth or that only one mode could be used at a time - but actually, we were just missing the "Bearer " prefix for our token

• my comment about the "Bearer " prefix (token not valid for running mode after SSO authentication #4991 (comment)) got a few upvotes, so thought this would be worth documenting as well

Modifications

• add a link to the auth error message in gatekeeper.go when the token is invalid

  • shorten the error message as well, per above motivation

• add the "Bearer " prefix as another common reason for this error in the FAQ

Verification

Ran make docs

[terrytangyuan]

docs build need to be fixed

[agilgur5]

hmm let me figure out which part broke -- looks like it may be detecting the Bearer reference as a false positive token match. from the logs:

-If you're using `--auth-mode=client`, make sure you have `*** in front of the token, as mentioned in [Access Token](access-token.md#token-creation).
+If you're using `--auth-mode=client`, make sure you have `Bearer` in front of the token, as mentioned in [Access Token](access-token.md#token-creation).

it's also missing the intentional space after Bearer in the docs as well 🤔

[agilgur5]

ah okay, looks like removing the intentional space in front of Bearer was the change (from markdownlint), and then GitHub Actions's log is redacting it as a false positive. (I thought something was off because I didn't see any specific secret checking steps in make docs, that makes more sense now that GitHub is doing that 😅 )

[agilgur5]

@terrytangyuan error should be fixed now

Seems like markdownlint has no line ignore functionality unfortunately (markdownlint/markdownlint#16), so just removed the intentional space. EDIT: that was the wrong markdownlint, this is in fact possible in the one we use: https://github.com/DavidAnson/markdownlint#configuration