release / release notes: Should clean up "Roadmap", "Downloads", and "API"

[EricCousineau-TRI]

• Rename "Roadmap" to "Releases", merge with Downloads somehow?
• Add a link to API Documentation once later versions are included? (or only publish for major versions???)
• API docs for 9.0.0 are broken? http://osrf-distributions.s3.amazonaws.com/sdformat/api/9.0.0/index.html
• API docs for "dev" seem old? http://osrf-distributions.s3.amazonaws.com/sdformat/api/dev/classes.html
• Add a link to Downloads if/when they're in sync with API Docs?
• Use GitHub release page?

[scpeters]

I just uploaded the 9.0.0 api docs by doing the following:

# from sdformat clone, with doxygen installed
git checkout sdformat9_9.0.0
mkdir build
cd build
cmake ..
make doc
cd doxygen/html
s3cmd sync . s3://osrf-distributions/sdformat/api/9.0.0/

[scpeters]

I think the dev API is copied from 6.0.0. what do we want that to point to? should it just point to the newest tag in the list (currently 9.0.0)?

[EricCousineau-TRI]

Yeah, newest tag in the list.
Also, ideally, the newest tag overal is at the top of the list. (Or if it isn't, the rationale is stated.)

[scpeters]

I just updated the dev api to match 9.0.0

[scpeters]

Use GitHub release page?

I created the following issue to discuss creating releases with descriptions as part of our release process: gazebo-tooling/release-tools#195