[ONP-3448] - Document disk image size

[imlogang]

https://linear.app/circleci/issue/ONP-3448/document-how-to-build-a-custom-containerdisk-image-for-runner

Description

Updated runner-provisioner docs to document the image size limit, and how to resize it.

Reasons

Why did you make these changes? What problem does this solve?

Content checks

Please follow our style when contributing to CircleCI docs. View our style guide or check out our CONTRIBUTING.md for more information.

Preview your changes:

• View the Vale linter results, select the ci/circleci: lint job at the bottom of your PR. You will be redirected to the vale/lint job output in CircleCI.
• Preview your changes, select the ci/circleci: build job at the bottom of your PR and you will be redirected to CircleCI. Select the Artifacts tab and select index.html to open a preview version of the docs site built for your latest commit.

Take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs):

Content structure:

• Break up walls of text by adding paragraph breaks.
• Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• Include relevant backlinks to other CircleCI docs/pages.

Formatting:

• Keep the title between 20 and 70 characters.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).

[linear-code]

ONP-3448