Adding documentation for the signin command

[rjkerrison]

Why:

Closes: GitHub&Slack #84162

What's being changed (if available, include any code snippets, screenshots, or gifs):

Adding a line of documentation for the command /github signin which is otherwise only discoverable by guesswork or by finding community threads such as GitHub&Slack #84162.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Pull request overview

This PR adds documentation for the /github signin command in the GitHub Slack integration documentation. The change provides users with information about a command that allows them to restart the GitHub account connection workflow within Slack.

Key changes:

• Added a new slash command entry to the command reference table documenting the /github signin command

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
integrations/how-tos/slack/use-github-in-slack.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Sharra-writes]

@rjkerrison This looks like a low-stakes addition with a pretty good amount of community testing, so I'll go ahead and accept it.

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