Styling nits

[aphillips]

The "Note Well" in Section 2.2 is now decorated the same way as the MUSTard. It should probably use a variation on the normal "Note" style.

The quote from UTR51 in Section 2.7 (emoji sequences) looks like it might have contrast issues incompatible with accessibility. We should probably make the gray background much lighter.

The new link-above style puts the [S][I][C] markers into the above link. They were previously inline and styled to stand out. Consider restoring that appearance?

[r12a]

1. Could do, or we could leave as is. The layout is different from mustard. Question is whether this looks like mustard, or whether mustard looks like "Note well", or whether it matters anyway.

2. yes

3. I did think about that. The new approach saved a lot of time on a conversion process that already took far longer than i was comfortable with. The cues are still as searchable as they were before, and i don't see a great difference in visibility. Generally, the text of the mustard also indicates, where necessary, who the audience is (which is good practice for quoting the mustard elsewhere). So i thought it was ok.

[aphillips]

1. occurs because "Note Well" and requirements both use the same style: advisement. In my PR I changed the "Note Well" to use the addition style, which is sort of squidgy and does produce an interesting shade of aubergine. See if you like it?

2. I fixed by making the gray much lighter.

3. I thought about and then left for further consideration. Suggest we split into another issue.