Add a legend of possible pipeline statuses and their icons

What does this MR do?

This MR adds a legend of possible pipeline statuses and the corresponding icons to the docs about pipelines. This was motivated by a conversation in a ticket 🎫 that is available to GitLab team members with access to ZenDesk. Being able to refer to a table like this would have aided the resolution of the ticket and would be generally useful.

Example

This is an example of the table.

Pipeline Status	Icon	Alt Text
pending		yellow "pause" sign
running		circle two-thirds filled with blue
passed		green checkmark in green circle
failed		red x in red circle
skipped
canceled		dark gray slash (top left to bottom right) in dark gray circle

❓ Questions for Technical Writing Team

• YES. Is copying the referenced CI favicons from app/assets/images/ci_favicons into doc/ci/img the best way to get the referenced images in the docs? (It seems like a reasonable approach but requires duplicating the images with in the code base and that seems suboptimal but not completely unreasonable.)

Screenshots (strongly suggested)

A screenshot of how this looks in the review app:

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

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