Fill up nav sections with next up links

fill.js

const walk = (a, index = 0, parent = null) =>
    a.forEach((e, i, arr) => {
        const fullpath = `~/ag-grid/documentation/ag-grid-docs/src/content/docs/${e.path}/index.mdoc`;
        let f = fs.readFileSync(fullpath).toString();
        let next = arr[i + 1];
        if (e.children) {
            next = e.children[0];
            walk(e.children, i, arr);
        }
        if (!next && parent && parent[index + 1]) {
            next = parent[index + 1];
        }
        if (!f.includes('## Next up') && next) {
            f += `## Next Up
Continue to the next section: [${next.title}](./${next.path}/).
`;
            fs.writeFileSync(fullpath, f);
        }
    });
walk([
    {
        type: 'item',
        title: 'Configuration',
        path: 'configuration',
        children: [
            { type: 'item', title: 'Column Definitions', path: 'column-definitions' },
            { type: 'item', title: 'Updating Definitions', path: 'column-updating-definitions' },
            { type: 'item', title: 'Column State', path: 'column-state' },
        ],
    },
    { type: 'item', title: 'Column Headers', path: 'column-headers' },
    { type: 'item', title: 'Column Groups', path: 'column-groups' },
    { type: 'item', title: 'Column Sizing', path: 'column-sizing' },
    { type: 'item', title: 'Column Moving', path: 'column-moving' },
    { type: 'item', title: 'Column Pinning', path: 'column-pinning' },
    { type: 'item', title: 'Column Spanning', path: 'column-spanning' },
]);