Set immediately_reserve parameter to true

[atovpeko]

Description

Bug fix on https://docs.timescale.com/self-hosted/latest/replication-and-ha/configure-replication/#create-replication-slots

Links

Fixes #3219

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[CLAassistant]

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[github-actions]

Allow 10 minutes from last push for the staging site to build. If the link doesn't work, try using incognito mode instead. For internal reviewers, check web-documentation repo actions for staging build status. Link to build for this PR: http://docs-dev.timescale.com/docs-3219-site-bug-issue-with-the-page-self-hostedlatestreplication-and-haconfigure-replication

[billy-the-fish]

LGTM