Add last modified date

[husseinalhammad]

This PR adds the last modified date of a document. Closes #33

This just reads the last modified date via the file system instead of querying GitHub. Not sure if this would get us any unexpected values. Worth checking the Netlify previews.

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Update: that did not work unfortunately (always get the day's date). Switching this to draft for now.

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Update: Date is now fetched from GitHub's public API. Example: https://api.github.com/repos/email-markup-consortium/emailmarkup.org/commits?path=src%2Fpages%2Fen%2Fdocs%2Findex.md&page=1&per_page=1

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[husseinalhammad]

Good catch @M-J-Robbins.

• The attribute value has been updated to ISO 8601 which is listed as a valid format.
• The date format has been updated to "day month year"