Add migration notes to reference guide

[simonbasle]

• polish: add anchor for observation, underline H5
• Add migration notes appendix with 3.5 migration notes

[simonbasle]

@reactor/team this is a first draft at more fleshed out migration notes from 3.4.x to 3.5.0.

I'm not sure about adding this to the reference guide. It could as well be a wiki page, which we can reference from the guide and the release notes.

the pros:

• can be reviewed using PRs
• community can suggest improvements using PRs

the cons:

• history of migration notes could pile up in the reference guide
• this represents a few pages more in the guide (especially in the future, considering the above)
• using 3.5.0 as an example: the notes have to be finalized before the release OR:
• edits won't be visible until the next patch release

wdyt?

[chemicL]

This is long overdue. As I believe users will still find this information helpful, I published this as a wiki: https://github.com/reactor/reactor-core/wiki/Migration-Guides.