docs(add): article on clean urls

documentation/content/Advanced Features/clean-url.md

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+---
+authors: Talha Mansoor
+Title: Clean URLs
+Tags: unique
+Date: 2019-08-10 20:44
+Slug: clean-urls
+Category: Advanced Features
+---
+
+Out of the box, Pelican generated URLs are [uncleaned](https://en.wikipedia.org/wiki/Clean_URL), i.e. they have `.html` appended. Elegant lets you use clean URLs with minimal configuration.
+
+!!! Important "Mandatory"
+
+    To use clean URLs, you server must support it.
+
+## Categories, Tags and Archives
+
+In your Pelican configuration set following values
+
+```python
+TAGS_URL = "tags"
+CATEGORIES_URL = "categories"
+ARCHIVES_URL = "archives"
+```
+
+With it set, your categories, tags and archives will be served from following links,
+
+1. [`/categories`](/categories)
+1. [`/tags`](/tags)
+1. [`/archives`](/archives)
+
+If you do not set the variables then the URLs will become
+
+1. [`/categories.html`](/categories.html)
+1. [`/tags.html`](/tags.html)
+1. [`/archives.html`](/archives.html)
+
+## Articles
+
+For clean URLs for articles, just set
+
+```python
+ARTICLE_URL = "{slug}"
+```
+
+## Pages
+
+For pages, set
+
+```python
+PAGE_URL = "{slug}"
+PAGE_SAVE_AS = "{slug}.html"
+```