ICU-22947 Update user guide for MF2

[catamorphism]

This PR updates the user guide for MF2, moving statements into the present tense instead of future tense. It also adds a section on ICU4C that, for now, links out to https://messageformat.dev .

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22947
• [x] Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• [x] Issue accepted (done by Technical Committee after discussion)
• [x] Tests included, if applicable
• [x] API docs and/or User Guide docs changed or added, if applicable