Update details about text formatting

What does this MR do?

This MR combines all the typographic details in the style guide into a new ## Text formatting section. They were scattered around the page and could use better headers, so now they are all collated together, and all follow a similar pattern to make it easier to follow. The examples are also standardized to better represent the expected usage and formatting.

Copy edits

• Clarify details about bold and backticks with better examples.

• Use both rendered examples, along with code blocks that demonstrate the markdown formatting.

• Recommend avoiding both italics and quotes, along with moving quotes section to punctuation, and moving <kbd> text formatting to the new text formatting section.

• For quotes, explaining to use the British/International style for quotes with punctuation:

  Put commas and periods outside the quotation marks unless the quotation is also a complete sentence or the punctuation is part of the quotation.

  As shown in the majority of pages, this is already our standard, for example:

  • https://gitlab.com/gitlab-org/gitlab/-/blob/a535cfcb9cb6d765cbb6c2e711bd2dc0df930482/doc/ci/migration/jenkins.md#L76
  • https://gitlab.com/gitlab-org/gitlab/-/blob/a535cfcb9cb6d765cbb6c2e711bd2dc0df930482/doc/ci/pipelines/downstream_pipelines.md#L34
  • https://gitlab.com/gitlab-org/gitlab/-/blob/a535cfcb9cb6d765cbb6c2e711bd2dc0df930482/doc/ci/yaml/yaml_optimization.md#L323

• This also cleans up some non-standard ("curly") quotation marks, but without looking too closely as to whether or not the quotes can be removed.

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