Document heading level config options for Timeline component

[robertjmccarthy]

What

Update Timeline component nunjuck macro options to reflect configurable heading levels.

Why

@aliuk2012 spotted that the Timeline component has a configurable heading option which is not documented within the components nunjuck macro options.

Link to Slack message on #moj-design-system-support Slack channel: https://mojdt.slack.com/archives/CH5RUSB27/p1733904532246179

Who needs to be involved

• Content Designer (leading)
• Developer (supporting)

Done

When the nunjuck macro option is updated to reflect the configurable heading level, and @aliuk2012 is notified of the update.

[helennickols]

Chris and I have agreed the following. This is the exact styling of capitals etc:
Name - headingLevel
Type - numeric
Required - No
Description - A number for the heading level. Defaults to 2 (

).

This follows the style in the Side nav (but we have added a full stop after the round brackets).

I'll do a PR now.

[helennickols]

@chrispymm PR please #1052