Add Join PrestoDB on Slack link on document pages

[yhwang]

Description

Adding a link in the document pages to encourage users to join the PrestoDB Slack when seeking help.

Motivation and Context

Document pages are where users look for instructions/information when setting up a Presto cluster.
Adding the link to direct users to the Slack channels when encountering any issues would
improve the adoption process.

Impact

No function impact

Test Plan

Generate the doc website and check the new link

Contributor checklist

• Please make sure your submission complies with our contributing guide, in particular code style and commit standards.
• PR description addresses the issue accurately and concisely. If the change is non-trivial, a GitHub Issue is referenced.
• Documented new properties (with its default value), SQL syntax, functions, or other functionality.
• If release notes are required, they follow the release notes guidelines.
• Adequate tests were added if applicable.
• CI passed.

Release Notes

Please follow release notes guidelines and fill in the release notes below.

== NO RELEASE NOTE ==

[yhwang]

highlight the new link in the screenshot below:

and the link goes to: https://communityinviter.com/apps/prestodb/prestodb

[steveburnett]

LGTM! (docs)

Pull branch, local doc build.

The link for PrestoDB Slack visible in the right column of links.

Clicking the link opens the Community Inviter link for the PrestoDB Slack.

[yhwang]

@steveburnett not sure why a doc-only PR would trigger those GH actions for code changes. I thought we update GH workflow to handle doc-only PR different.

[steveburnett]

@steveburnett not sure why a doc-only PR would trigger those GH actions for code changes. I thought we update GH workflow to handle doc-only PR different.

I think the doc-only PR workflow may be limited to .rst files not .html, or may not include the /_templates directory.

[yhwang]

I believe the filter is:

          codechange:
            - '!presto-docs/**'

but seems the GH actions are not stable at this moment. Maybe someone is fixing it now.

[yhwang]

@steveburnett I believe there is something wrong with the GH workflow. Create a PR to fix it