Docs - Add `navbar-center` in navbar

[biewdev]

Example:

		<nav class="navbar">
			<div class="navbar-menu">
				<div class="navbar-section">
					<a href="#" class="navbar-item">Features</a>
					<a href="#" class="navbar-item">Plans</a>
					<a href="#" class="navbar-item">Server List</a>
					<a href="#" class="navbar-item">Blog</a>
					<a href="#" class="navbar-item">Help</a>
				</div>
				<div class="navbar-center">
					<a href="#">
						<img src="/img/logo.png" width="48" />
					</a>
				</div>
				<div class="navbar-section">
					<div class="navbar-item">
						<button class="btn is-primary mr-3">Sign In</button>
						<button class="btn is-dark">Sign Up</button>
					</div>
				</div>
			</div>
		</nav>