The documentation for how to create nested sidebars was unclear, and I could not find it anywhere else. After experimenting, I discovered the tactic and felt it should be in the docs.

```diff
+Gold
-Glod
```

Closes #614

Closes #625

- Change "What is it" heading to "What it is", as it's not a question
- Expand SSR acronym
- Slightly reword some passages for clarity

Fix #521

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Fix #577

- Ignore images outside of main content area (e.g. logo, sidenav icons, etc.)
- Ignore linked images (previous behavior broke linked images)

Fixes #520

CSS style for functions is specified in buble.styl only. Thus, for example, bash commands from prism-bash.js (function section) are not highlighted. You can add a new CSS style with the right color you think fits better  :).

### Issue

<img width="491" alt="screenshot 2018-10-30 at 23 31 35" src="https://user-images.githubusercontent.com/8396515/47757020-58587080-dc9c-11e8-867c-dc3feabcb651.png">

Sorry, forgot to include this line in previous PR.

-----
Please makes sure these boxes are checked before submitting your PR, thank you!

* [x] Make sure you are merging your commits to `master` branch.
* [x] Add some descriptions and refer relative issues for you PR.
* [x] DO NOT include files inside `lib` directory.

Allow user to configure whether or not to hide other sidebar content while showing searching results.

Related issue: #535