Add Video formats (video_frames and video_keyframes) (#872)

* initialization

* add examples

* refine descriptions of video format

* update base url

* update doc for video format

* correct typo

---------

Co-authored-by: wonjuleee <wonju@intel.com>

site/content/en/docs/formats/video.md

@@ -0,0 +1,69 @@
+---
+title: 'Video'
+linkTitle: 'Video'
+description: ''
+---
+
+## Format specification
+There are many kinds of video extensions as listed up
+[here](https://github.com/openvinotoolkit/datumaro/blob/develop/datumaro/plugins/data_formats/video.py).
+
+Datumaro can import a video into video frames by adjusting the start frame, end frame,
+and step size. Furthermore, with a `video_keyframes` format, Datumaro can extract
+keyframes by comparing zero-mean normalized cross correlation (ZNCC) metric between
+successive frames as following [here](https://www.sciencedirect.com/science/article/pii/S1047320312000223).
+Plus, Datumaro provides the options for choosing image extension and name patterns
+for efficient data management from multiple videos.
+
+## Import video
+
+A Datumaro project with a video frames can be created
+in the following way:
+
+```
+datum create
+datum import -f video_frames <path_to_video>
+```
+
+Load video through the Python API:
+
+```python
+import datumaro as dm
+
+dataset = dm.Dataset.import_from('<path_to_video>', format='video_frames')
+```
+
+Datumaro has few import options for `video_frames` format, to apply them
+use the `--` after the main command argument.
+
+`video_frames` import options:
+- `--subset` (string) - The name of the subset for the produced
+  dataset items (default: none)
+- `-p, --name-pattern` (string) - Name pattern for the produced
+  images (default: `%06d`)
+- `-s, --step` (integer) - Frame step (default: 1)
+- `-b, --start-frame` (integer) - Starting frame (default: 0)
+- `-e, --end-frame` (integer) - Finishing frame (default: none)
+- `-h, --help` - Print the help message and exit
+
+Usage:
+
+``` console
+datum import -f video_frames [-h] [-n NAME] [-r RPATH] [-p PROJECT_DIR]
+  [--no-check] [-s STEP] [-b START_FRAME] [-e END_FRAME] url ...
+```
+
+Example: import a video into frames, use each 30th frame:
+```console
+datum import -f video_frames video.mp4 -o video-frames --step 30
+```
+
+Example: import a video into frames, save as 'frame_xxxxxx.png' files:
+```console
+datum import -f video_frames video.mp4 -o video-frames --image-ext=.png --name-pattern='frame_%%06d'
+```
+
+Example: import a video into keyframes:
+```console
+datum import -f video_keyframes video.mp4 -o video-frames
+```