You can achieve that with custom server,

const express = require('express')
const next = require('next')

const header = fs.readFileSync(path.resolve(ROOT + '/resources', 'header.txt'), 'utf8');
const footer = fs.readFileSync(path.resolve(ROOT + '/resources', 'footer.txt'), 'utf8');

const port = parseInt(process.env.PORT, 10) || 3000
const dev = process.env.NODE_ENV !== 'production'
const app = next({ dev })
const handle = app.getRequestHandler()

app.prepare().then(() => {
  const server = express()

  server.all('*', async (req, res) => {
    const nextResponse = await handle(req, res);
    return mergeHTML(header, footer, nextResponse); // You will need to "merge" cause nextResponse has `head` & `body` as well.
  })

  server.listen(port, err => {
    if (err) throw err
    console.log(`> Ready on http://localhost:${port}`)
  })
})

mergeHTML should merge head from your header & next's head & the body. You can use cheerio to help you with that.

Here's a simple example:

import React from 'react';

const parseText = (text) => {
  const lines = text.split('\n');
  let parsing = {
    title: '',
    content: [],
  };

  let currentKey = '';
  lines.forEach((line) => {
    var line = line.trim();
    if (line !== "") {
      if (line.startsWith('#')) {
        currentKey = line.substring(1).toLowerCase();
      } else if (currentKey === 'title') {
        parsing.title = line;
      } else if (currentKey === 'content') {
        const formattedLine = line.replace(/\\b(.+?)\\b/g, '<strong>$1</strong>')
          .replace(/\\i(.+?)\\i/g, '<em>$1</em>');
        parsing.content.push(formattedLine);
      }
    }
  });

  return parsing;
};

const TextParser = ({ text }) => {
  const parsing = parseText(text);
  console.log(parsing)
  return (
    <div>
      <h1>{parsing.title}</h1>
      <div>
        {parsing.content.map((line, index) => (
          <p key={index}>{line }</p>
        ))}
      </div>
    </div>
  );
};

export default TextParser;

With this component, you can use it like this:

import React from 'react';
import TextParser from './TextParser';

const App = () => {
  const text = `
    #title
    A Title
    #content
    A first \\b bold\\b line
    A second \\i italic\\i line
  `;

  return (
    <div>
      <TextParser text={text} />
    </div>
  );
};

export default App;

The resulting HTML looks like:

<div>
  <h1>A Title</h1>
  <div>
    <p>A first <strong> bold</strong> line</p>
    <p>A second <em> italic</em> line</p>
  </div>
</div>

This approach assumes a simplified case, and you may need to expand or adjust the parsing logic based on your specific needs.