Post-docsy migration tasks

Followup to:

- #746

Site links:

- **Docsy site** is live in production https://www.jaegertracing.io
- **Legacy site**: https://legacy--jaegertracing.netlify.app

Hugo config & site pages:

- [x] Simplify and consolidate - #1025
- [ ] Enable Hugo's default [robots.txt](https://gohugo.io/templates/robots/)
- [ ] Drop pre-docsy front matter that is no longer needed, such as `hasParent`, etc.
- [ ] .

Netlify:

- [x] Remove `docsy` branch config - https://docsy--jaegertracing.netlify.app
- [x] Setup legacy branch config - https://legacy--jaegertracing.netlify.app

Other:

- [ ] Upstream relevant local Docsy layout customization to Docsy repo
- [ ] Upgrade Docsy
- [x] Trigger site reindexing by Google. Done via sitemap.xml section.
- [x] Ensure legacy branch has robots meta set to `noindex,nofollow`
- [x] Add @yurishkuro and Johan to the [site's Google Search console](https://search.google.com/search-console/users?resource_id=sc-domain%3Ajaegertracing.io) member list as owners

Extra (nice to have, maybe later):

- [ ] Prepare for https://github.com/jaegertracing/jaeger/issues/6321 by, e.g., renaming config var from v2 to base, etc.
- [ ] Consider not fetching the blog posts data on every build?
- [ ] Switch to Hugo [config dir](https://gohugo.io/configuration/introduction/#configuration-directory) rather than our custom WKSP feature

### Tasks

- [x] #1060
- [x] #1061
- [x] #1062
- [ ] ...

Start with the Hugo config and site pages, then review the unchecked items in this migration checklist, including robots.txt, obsolete front matter, Docsy customization, and the Docsy upgrade. Done means completing clearly selected checklist items and updating the issue with their status; the issue does not identify specific files or tests.

Written by the indexing model from the issue text.