Document organization deletion feature

[yaningo]

Description

• Added a new page to document organization deletion.
• Moved project deletion content from the Stop building a project on CircleCI page to the newly created page.
• Updated note on the "Stop building a project on CircleCI" page with an xref to the new location of the project deletion content.
• Added navigation item (under"Manage security and secrets" > "How-to guides") pointing to the new content.

Reasons

• Organization deletion is available but is not documented.
• Regrouping deletion-related content (for both organizations and projects) on a single page makes more sense (up for debate 😅 )

Left to do

• Add relevant notes to the table on the newly created page.
• Update the Roles and permissions matrix to include organization and project deletion.
• Fix the column title alignment in all tables of the aforementioned matrix.

Content Checklist

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Please 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):

• 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.
• Keep the title between 20 and 70 characters.
• Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).
• Include relevant backlinks to other CircleCI docs/pages.