About/why text messaging content

[heyitsmebev]

As part of the new About Notify content section, this PR is the implementation for "Why text messaging"

Definition of Done

• Should include a side navigation
• Should be able to easily navigate to About Notify content
• Should meet a11y standards

Figma: https://www.figma.com/proto/aAWhlPzOP6kfVFnCCAfSZZ/notify.gov?page-id=2339%3A3817&node-id=5392-875&node-type=frame&viewport=2870%2C-701%2C0.48&t=nfkxOXnOCIFQB1yK-1&scaling=min-zoom&content-scaling=fixed&starting-point-node-id=5392%3A875

Content: https://docs.google.com/document/d/1lP8I0Jxu0B26cmTGBQcoHCbfSvKKHQBE6gzTL9HUheE/edit?tab=t.0#heading=h.icyigvoln8tg

[jonathanbobel]

@heyitsmebev - Why text messaging and Security are both sub-pages of About Notify FYI. Example:

(and we're moving Join Notify and Contact to the top nav, I mentioned this in another issue)