Separated plugin documentation from extensions (#1390)

* Create minify-html.md

* Rename docs/minify-html.md to docs/plugins/minify-html.md

* Create search.md

* Update and rename docs/extensions/revision-date.md to docs/plugins/revision-date.md

* Update getting-started.md

* Update mkdocs.yml

docs/plugins/minify-html.md

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+# Minify HTML
+
+[mkdocs-minify-plugin][1] is an extension that minifies HTML by stripping all whitespace from the generated documentation.
+
+  [1]: https://github.com/byrnereese/mkdocs-minify-plugin
+
+## Installation
+
+Install the plugin using `pip` with the following command:
+
+``` sh
+pip install mkdocs-minify-plugin
+```
+
+Next, add the following lines to your `mkdocs.yml`:
+
+``` yaml
+plugins:
+  - search
+  - minify:
+      minify_html: true
+```
+
+!!! warning "Remember to re-add the `search` plugin"
+
+    If you have no `plugins` entry in your config file yet, you'll likely also
+    want to add the `search` plugin. MkDocs enables it by default if there is
+    no `plugins` entry set.
+
+## Usage
+
+The output is automatically minified by the plugin.

docs/plugins/revision-date.md

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+# Revision date
+
+[mkdocs-git-revision-date-localized-plugin][1] is an extension that shows the
+date on which a Markdown file was last updated in _Git_ at the bottom of each
+page. The date is extracted at the time of the build, so `mkdocs build` must
+be triggered from within a Git repository.
+
+  [1]: https://github.com/timvink/mkdocs-git-revision-date-localized-plugin
+
+## Installation
+
+Install the plugin using `pip` with the following command:
+
+``` sh
+pip install mkdocs-git-revision-date-localized-plugin
+```
+
+Next, add the following lines to your `mkdocs.yml`:
+
+``` yaml
+plugins:
+  - search
+  - git-revision-date-localized
+```
+
+!!! warning "Remember to re-add the `search` plugin"
+
+    If you have no `plugins` entry in your config file yet, you'll likely also
+    want to add the `search` plugin. MkDocs enables it by default if there is
+    no `plugins` entry set.
+
+## Usage
+
+The date is automatically added at the bottom of each page, e.g.:
+
+```
+Last updated: 9 December, 2019
+``` 
+
+### Changing the language
+
+The date is printed according to the locale which is determined through the
+[theme language][2] that was set in `mkdocs.yml`.
+
+  [2]: https://squidfunk.github.io/mkdocs-material/getting-started/#language
+
+### Changing the format
+
+To change the date format, set the `type` parameter to one of `date`,
+`datetime`, `iso_date`, `iso_datetime` or `timeago`, i.e.:
+
+``` gnuplot
+28 November, 2019                  # type: date
+28 November, 2019 13:57:28         # type: datetime
+2019-11-28                         # type: iso_date
+2019-11-28 13:57:26                # type: iso_datetime
+20 hours ago                       # type: timeago
+```
+
+Example:
+
+``` yaml
+plugins:
+  - git-revision-date-localized:
+      type: timeago
+```
+
+Result:
+
+```
+20 hours ago
+```

docs/plugins/search.md

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+# Search
+
+MkDocs enables the search plugin by default if there is no `plugins` entry set in `mkdocs.yml`. If additional plugins are installed, the `search` plugin must be added to your `mkdocs.yml`. See [Site search][1] for more information about how to use search with Material.
+
+  [1]: ../getting-started.md#site-search
+
+## Installation
+
+Add the following lines to your `mkdocs.yml`:
+
+``` yaml
+plugins:
+  - search
+```
+
+!!! warning "Remember to re-add the `search` plugin"
+
+    If you have no `plugins` entry in your config file yet, you'll likely also
+    want to add the `search` plugin. MkDocs enables it by default if there is
+    no `plugins` entry set.