Left navigation bar - Add horizontal divider style (change won't be visible until someone adds the divider)

[lenasano]

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Description

• Please describe your changes: Add styling for left nav menu horizontal divider / design source: Figma file
• Jira/GitHub Issue numbers (if any): N/A
• Preview links/images (Internal contributors only): See example New to IaCM? divider on IaCM page / See also comment below.

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[bot-gitexp-user]

Current Draft URL is: https://6749c0cfcdc2ec8ba6f82741--harness-developer.netlify.app

Note to reviewer:
This preview shows the left nav menu horizontal divider at this link: https://6749c0cfcdc2ec8ba6f82741--harness-developer.netlify.app/docs/infrastructure-as-code-management

[bot-gitexp-user]

Current Draft URL is: https://6749ce19e88c34b0cbe3fe4b--harness-developer.netlify.app

☝🏼 This (latest) revision does not show any left nav menu horizontal dividers in any module. Instead, this PR adds the css styling so that any module can add a divider if they would like to.

[rohanmaharjan100]

@lenasano Everything looks great! How about adding style for dark mode as well?

[bot-gitexp-user]

Current Draft URL is: https://674c6187e6d0f6848e6bdf61--harness-developer.netlify.app

@rohanmaharjan100 You can see dark mode now as well, at https://674c6187e6d0f6848e6bdf61--harness-developer.netlify.app/docs/infrastructure-as-code-management/

[bot-gitexp-user]

Current Draft URL is: https://674c6245cdc2ec85c4f82626--harness-developer.netlify.app

☝🏼 This is the latest link, again without visual changes.