Hide data and metadata pages from sidebar

[MathewBiddle]

These two pages overlap with other pages on the site. We should consider removing:

• https://ioos.github.io/mbon-docs/metadata.html
• https://ioos.github.io/mbon-docs/data.html

[MathewBiddle]

@laurabrenskelle do you mind taking a look at this? These two pages are old and could be removed. If you agree, mind taking a stab at trying to hide them someway?