Fix sub section links for /docs/gatsby-config

[siddharthkp]

Bug:

The links for configuration options on the Gatsby Config page don't scroll to the right section because they have incorrect id

URL in link: https://www.gatsbyjs.org/docs/gatsby-config/#pathPrefix
Actual link: https://www.gatsbyjs.org/docs/gatsby-config/#pathprefix

Notice the lowercased part at the end. The actual links are generated by gatsby-remark-autolink-headers.

Fix

In this pull request, I've changed the url in links to be lowercase as well. (This is what the react docs do as well)

Alternate Fix

The problem with this fix is that it's not consistent with the older version of the docs which use camelCased links. Example: https://www.gatsbyjs.org/docs/actions/#addThirdPartySchema

gatsby-remark-autolink-headers uses github-slugger to create the slugs for markdown headers - which supports camelCased slugs as well by passing an addition argument (test case: github-slugger/index#13)

This will have to be opt-in option for gatsby-remark-autolink-headers so that applications already using it don't break.

Let me know what you think

[pieh]

Thanks @siddharthkp!

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