Update GitLab Pages multiple deployments docs to remove conflicting statements

Problem to solve

Update GitLab Pages multiple deployments documentation with reference to recent development.

Reference conflicting statements:

In this section, it is mentioned:

Leading and trailing hyphens (-) are removed.

And in this example, documentation is using leading and trailing hyphens for pages.path_prefix.

PAGES_PREFIX: '-stg-'
pages.path_prefix: '-${CI_MERGE_REQUEST_IID}-'

Further details

Proposal

Who can address the issue

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