Complete pathogen_marburg vignette

[cm401]

Need to add:

• [x] Introduction section with background and placeholder for link to the pre-print / paper
• [x] outbreak table
• [x] delay table
• [x] seroprevalence table
• [x] panel plot
• [x] CFR plot
• [x] quality assessment plot
• [x] risk factor table
• [x] commentary for each section / captions