[docs]: Overhaul user data formats page

[holmanb]

Cloud-init supports multiple formats, which are described on the user data formats page.

This page has a few improvements that should be made:

1. Each format deserves a very brief example. This will make our format page more readable and allows skimming and reader intuition to fill in details faster than reading a wall of text.
2. This page contains an unrelated kernel command line section. This section does not describe a format, and should be relocated or (more likely) deleted.
3. This page contains an unrelated how to disable user data section. This section does not describe a format, and should be relocated.
4. This page contains a how to write your own part-handler section. Future plans for this section have already been described in another issue. tl;dr - this belongs elsewhere
5. Jinja templated user data is not documented on this page (it is best described on the instance metadata page)
6. Currently these formats are not categorized, but could be categorized to make them easier to skim / grok by the reader (see the next section).

Recommended format categorization

These formats fall into two categories, which I will call base formats and meta-formats for now.

Base formats are formats that allow the user to configure the instance via cloud-init. Meta formats are formats that modify or aggregate base formats.

Base formats include the following: cloud config data, user data scripts, cloud-boothooks

Meta formats include the following: include files, gzip compressed content, mime archives, jinja templates (need to be documented)

[TheRealFalcon]

The User Data Scripts section should additionally:

• Remove euca-run-instances reference
• Provide info on run frequency
• Include jinja templating in an example to show how that may be used in shell scripts