Redirect devel and 2.6 to new maas.io/docs

[nottrobin]

Remove 2.6, devel and redirect those places to maas.io/docs

(this will be published to old-docs.maas.io when we go live with maas.io/docs)

QA

./run build
docker build --build-arg REVISION_ID=none --tag maas-docs .
docker run -ti -p 8996:80 maas-docs

Now check redirects work:

• [x] http://127.0.0.1:8996/ -> https://maas.io/docs
• [x] http://127.0.0.1:8996/2.6 -> https://maas.io/docs
• [x] http://127.0.0.1:8996/2.6/en -> https://maas.io/docs
• [x] http://127.0.0.1:8996/devel/en/ -> https://maas.io/docs
• [x] http://127.0.0.1:8996/devel/en/somewhere -> https://maas.io/docs/somewhere
• [x] http://127.0.0.1:8996/2.5 -> http://127.0.0.1:8996/2.5/en/
• [x] http://127.0.0.1:8996/2.5/en/ loads
• [x] Other pages in nav load