Redirect previous roadmap location to current one

[davidhorstmann-arm]

The document roadmap.md was linked-to several times then moved as part of restructuring work. In order to retain the working link, add the sphinx-reredirects module and tell it to redirect the old roadmap.md location to the new one.

[davidhorstmann-arm]

Note for reviewers: The result on this PR's build of the docs is that the old location:
https://mbed-tls--90.org.readthedocs.build/en/90/roadmap/
Is redirected to the new location:
https://mbed-tls--90.org.readthedocs.build/en/90/project/roadmap/

[bensze01]

So far I've been adding redirects to the readthedocs config - which produces HTTP 301 moved redirects rather than HTML <meta> redirects like sphinx-reredirects. The downside is that the list of redirects lives outside this repo, but it is more compatible with non-browser clients.

[bensze01]

The relevant docs: https://docs.readthedocs.io/en/stable/user-defined-redirects.html

[bensze01]

FYI: Readthedocs feature request for managing redirects in-tree, with potential workarounds listed: readthedocs/readthedocs.org#4221

[davidhorstmann-arm]

So far I've been adding redirects to the readthedocs config - which produces HTTP 301 moved redirects rather than HTML <meta> redirects like sphinx-reredirects. The downside is that the list of redirects lives outside this repo, but it is more compatible with non-browser clients.

Ah okay, that makes more sense then. I'll close this PR as superfluous.