[docs] Anchors not rendered as intended in the github pages

[softark]

In #8168, a link (db-dao.md#database-schema) has been modified as (db-dao.md#working-with-database-schema-). IMO, it should be reverted, since the intended anchor name in db-dao.md is 'database-schema'.

But it seems that the modification has been made based on the fact that the anchors are not rendered as intended in the github pages.

For example, this is what we have in http://www.yiiframework.com/doc-2.0/guide-db-dao.html

<h2>
Working with Database Schema 
<span id="database-schema"></span> 
<a href="#working-with-database-schema"
  id="working-with-database-schema"
  class="hashlink">&para;</a>
</h2>
<p>Yii DAO provides ...

But the counterpart of it in https://github.com/yiisoft/yii2/blob/master/docs/guide/db-dao.md is:

<h2>
<a id="user-content-working-with-database-schema-" 
  class="anchor" 
  href="#working-with-database-schema-" 
  aria-hidden="true">
  <span class="octicon octicon-link"></span>
</a>
Working with Database Schema
</h2>
<p>Yii DAO provides ...

(Note that I've reformatted the htmls for the sake of readability)

As you see, <span id="database-schema"></span> is lost in the github version. And the auto created elements are different from each other.

As far as I understand, the policy in the Yii documentation is to use <span id="anchor-name"></span> for an anchor. And I do favor it, because it is easy to refer to from other places of the documentation.

Especially, the auto created anchors based on the heading's text could be a nightmare for multi-byte languages: they will be URL-encoded. For example, https://github.com/yiisoft/yii2/blob/master/docs/guide-ja/db-dao.md goes like the following:

<h2>
<a id="user-content-データベーススキーマを扱う-" 
  class="anchor"
  href="#%E3%83%87%E3%83%BC%E3%82%BF%E3%83%99%E3%83%BC%E3%82%B9%E3%82%B9%E3%82%AD%E3%83%BC%E3%83%9E%E3%82%92%E6%89%B1%E3%81%86-"
  aria-hidden="true">
  <span class="octicon octicon-link"></span>
</a>
データベーススキーマを扱う 
</h2>
<p>Yii DAO は、...

It will be very hard for us to use and maintain such an anchor in the documentation source text.

Although the fact is that the anchors in the span tags are not rendered as intended in the github pages, and I think that we have to deal with the following problems:

1. In the github pages, the anchors won't work. This is inconvenient even if you are using them just for previewing purpose.
2. In the Yii site, we don't have the documentations of apps and extensions yet. As for now, the links to them from the Yii 2 guide can do nothing but point to the github pages.

[yii-bot]

Issue moved to yiisoft/yii2-apidoc#39