The simplest way I can find to find a sorted list of all tags across all posts in Siteleaf.

tags.html

{% capture tags %}
 	{% for post in site.posts %}
  		{% for tag in post.taxonomy.tags %}
			{{tag.value}}\{{tag.url}}{% unless forloop.last %},{% endunless %}
		{% endfor %}
	{% endfor %}
{% endcapture %}

{% assign tags = tags | strip_newlines | split:',' | sort %}

<ul>
{% for tag in tags %}
	{% assign tag_parts = tag | split:'\' %}

	{% unless last_tag == tag_parts.first %}
		<li><a href="{{tag_parts.last}}">{{tag_parts.first}}</a></li>
	{% endunless %}

	{% assign last_tag = tag_parts.first %}
{% endfor %}
</ul>

1. In Siteleaf you can call your tag set anything you want (default is 'tags', but can be anything). values is the generic name for the array of 'tag' values. Here is what the data looks like behind the scenes:

https://github.com/siteleaf/siteleaf-api#get-v1taxonomyidjson

We'll update the theme docs to make this more clear.

2. Agreed, they list it but don't explain it further.

3. You can't sort on a for statement, but you can sort first and assign it to a new variable like this:

{% assign sorted_tags = site.pages['blog'].taxonomy['tags'].values | sort:'value' %}

Now you can use the sorted_tags variable:

{% for tag in sorted_tags %}
    <li><a href="{{tag.url}}">{{tag.value}}</a></li>
{% endfor %}