Issue 72: Split why it works vignette into intro and analytic solutions vignette

[SamuelBrand1]

Description

This PR closes #72 and this PR closes #91 .

In this PR I have done two things:

1. Split the "Why it works" vignette away from the analytic solutions, which are now in their own vignette.
2. Beefed up the comparisons to the "Why it works" approach and both the generative model described in "Getting started" and the work of @parksw3 . I've acknowledged @parksw3 for writing out the direct link, which I've add a bit more explanation to.

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• [x] I have read the contribution guidelines.
• [x] I have tested my changes locally.
• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] My code follows the established coding standards.
• [x] I have added a news item linked to this PR.
• [x] I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.76%. Comparing base (2e7765f) to head (2ae9be0). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #106 +/- ## ======================================= Coverage 99.76% 99.76% ======================================= Files 10 10 Lines 427 427 ======================================= Hits 426 426 Misses 1 1 ``` | [Flag](https://app.codecov.io/gh/epinowcast/primarycensoreddist/pull/106/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=epinowcast) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/epinowcast/primarycensoreddist/pull/106/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=epinowcast) | `99.76% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=epinowcast#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.