feat(nimbus): add Learn more link to population sizing estimate UI

[yashikakhurana]

Because

• The automated population sizing estimate on the Audience page gave no indication of what targeting attributes are included in the count and what are excluded. Amber raised that owners need a way to understand what the estimate covers (channel, region/locale, version, standard targeting) and what it doesn't (Advanced Targeting event-based conditions, Required/Excluded Experiments), so they can interpret the number correctly.

This commit

• Adds a population_sizing_learn_more_url constant to AUDIENCE_PAGE_LINKS pointing to the population sizing docs section that explains inclusions/exclusions.
• Adds a "Learn more" link next to the "Audience Size Estimate" heading on the edit audience page.
• Adds a "Learn more" link next to the card title on the experiment detail sizing card.
• Adds a "Learn more" link beside the "estimated eligible clients" count on the rollout population sizing card.

Fixes #17098

[yashikakhurana]