Show mobile navigation menus only if links are present

[ang-zeyu]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• Others, please explain: Use portals for mobile site and page nav #1556 discussion follow up

Overview of changes:

• Change the checks for mobile navigation menus to require links to be present
• Flourish the documentation a little more so this note is clearer

Anything you'd like to highlight / discuss:
na

Testing instructions:

8 cases, all testable in the formattingContents.md / any other page with both site + page nav:
(also in snapshot tests)

// Edit layouts/userGuide.md to have:

// Case 1 & 2
// **with** and **without** wrapping id="site-nav"
// mobile nav menu button does not show up
<site-nav>
no links here
</site-nav>

// Cases 3 & 4
// Case 3 (with) also testable with a simple <a /> tag
// **with** and **without** wrapping id="site-nav"
// mobile nav menu button shows up
<site-nav>
* [**Getting Started**]({{baseUrl}}/userGuide/gettingStarted.html)
</site-nav>

// Cases 5 & 6
// Case 5 (with) also testable with a simple <a /> tag
// **with** and **without** wrapping id="page-nav"
// mobile page nav button shows up
{{ pageNav }}

// Cases 7 & 8
// Case 
// **with** and **without** wrapping id="page-nav"
// mobile page nav button does not show up
{{ pageNav }} // however, erase all headings in formattingContents.md

Proposed commit message: (wrap lines at 72 characters)
Hide mobile navigation menus if there are no links

---

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No blatantly unrelated changes
• Pinged someone for a review!

[jonahtanjz]

Nice work 👍 I've tried out the changes and everything seems to be working. LGTM :)