Add eleventyConfig.getFilters and eleventyConfig.getShortcodes to configuration docs

[zachleat]

#3310 (comment)

Docs for this currently exist here: https://www.11ty.dev/docs/languages/custom/#access-to-existing-filters-and-shortcodes but also need to be (arguably):

• https://www.11ty.dev/docs/config/
• https://www.11ty.dev/docs/create-plugin/