Make GL SVGs available to use in docs markup

This gives us the ability to add icons to our documentation using markup.

Supported SVGs: https://gitlab-org.gitlab.io/gitlab-svgs/

Usage:

Icon with default size (16px):

**{some-icon}**

Icon with custom size:

**{some-icon, 32}**

Icon with custom size and class:

**{some-icon, 32, some-class}**

Example:

**Default size:** **{admin}** (16px)

**Custom size:** **{admin, 8}** **{admin, 10}** **{admin, 12}** **{admin, 14}** **{admin, 16}** **{admin, 18}** **{admin, 24}** **{admin, 32}** **{admin, 48}** **{admin, 72}** (standard sizes: 8, 10, 12, 14, 16, 18, 24, 32, 48, 72)

**Custom class:** **{comment-next, 32, red}** **{tanuki, 32, custom-tanuki}** **{check-circle-filled, 32, background-green}**

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TODO:

• Add all standard icon sizes.
• Add the ability to define an icon size via markup.
• Add the ability to define custom icon classes via markup.
• Ping docs team to determine a markup format.

Closes #384 (closed)