Add heading-based navigation to LFX program intro pages (jump to term + back to top)

### Description The LFX program intro pages https://layer5.io/programs/lfx/2026 (e.g. `src/collections/programs/lfx-2026/lfx-2026.mdx`) are long and organized by headings (Spring/Summer/Fall). We need in-page navigation so users can jump to the internship term of interest and then return back to the top. #### Problem - Users must scroll to find the internship term they want. - After reading a term section, there’s no obvious “Back to top” action. ### Expected Behavior - Add an “On this page” navigation near the top of the intro page that links to the term headings present on that page (Spring / Summer / Fall). - Add a “Back to top” link near the end of each term section (or provide an always-available back-to-top control). - Ensure the anchor targets are stable and work reliably with Gatsby/MDX heading rendering. ### Screenshots <!-- Add screenshots, if applicable, to help explain your problem. --> ### Environment: - Host OS: - Browser: --- <img src="https://raw.githubusercontent.com/layer5io/layer5/master/.github/assets/images/layer5/5-light-small.svg" width="24px" align="left" /><h2>Contributor Resources and <a href="https://layer5.io/community/handbook">Handbook</a></h2> The layer5.io website uses Gatsby, React, and GitHub Pages. Site content is found under the [`master` branch](https://github.com/layer5io/layer5/tree/master). - 📚 See [contributing instructions](https://github.com/layer5io/layer5/blob/master/CONTRIBUTING.md). - 🎨 Wireframes and [designs for Layer5 site](https://www.figma.com/file/5ZwEkSJwUPitURD59YHMEN/Layer5-Designs) in Figma [(open invite)](https://www.figma.com/team_invite/redeem/GvB8SudhEOoq3JOvoLaoMs) - 🙋🏾🙋🏼 Questions: [Discussion Forum](https://discuss.layer5.io) and [Community Slack](https://slack.layer5.io). <img src="https://raw.githubusercontent.com/layer5io/layer5/master/.github/assets/images/buttons/community.webp" height="22px" align="left" />Join the Layer5 Community by submitting your [community member form](https://layer5.io/newcomer).