table of contents sometimes missing first heading tag

[thescientist13]

Type of Change

Bug

Summary

Observed in ProjectEvergreen/www.greenwoodjs.dev#129 that on this page, the first table of contents header was missing 🫨

Details

This was actually something I noticed in a previous set of patches when working on the website but looks like I forget to carry it over. The main issue being that for some reason we are just (seemingly) arbitrarily calling shift with no arguments?
https://github.com/ProjectEvergreen/greenwood/blob/v0.30.0/packages/cli/src/lifecycles/graph.js#L233

// parse markdown for table of contents and output to json
customData.tableOfContents = toc(fileContents).json;
customData.tableOfContents.shift();

// parse table of contents for only the pages user wants linked
if (customData.tableOfContents.length > 0 && customData.tocHeading > 0) {
  customData.tableOfContents = customData.tableOfContents
   .filter((item) => item.lvl === customData.tocHeading);

Seems like it has always been there and is causing some sort of race condition / unexpected behaviors, and I cannot for the life of me think of why it should be that way.