docs(examples): use button for navbar dropdown toggle

[TheDerbiedOne]

Fixes #40995

Updates the navbar examples to use a <button class="nav-link dropdown-toggle" type="button"> instead of an <a href="#"> for the dropdown toggle, avoiding docs that recommend an unsemantic placeholder link.

[fulldecent]

Thank you for sharing.

Can you please confirm that you have exhaustively searched the entire code base and that no a.dropdown-toggle remain?

[TheDerbiedOne]

Thanks — I double-checked.

I searched the repo for <a ... dropdown-toggle ...> and there are still a number of occurrences (including in tests and other docs/examples). This PR intentionally keeps scope narrow to the navbar docs example discussed in #40995, replacing that dropdown toggle anchor with a semantic <button class="nav-link dropdown-toggle" type="button" ...>.

If you’d like, I can follow up with a separate PR to convert the remaining docs examples (and/or tests), but it’d be a broader sweep than this single-example fix.