update doxy tag

[oxarbitrage]

Step 11 of https://github.com/bitshares/bitshares-core/wiki/Git-Flow#how-to-create-a-release states to manually update tag locally to generate new docs however this change is never ported back to any branch.

I think the doxy tag can be updated now IMO in develop as we now know:

• Next release will be named 3.1.0.
• Next release will be a feature release, in a hardfork one this update should be probably be made in hardfork branch.
• Current develop branch is not 3.0.0 anymore, but actually it is evolving into 3.1.0.

If agreement, i will update the wiki too.

[pmconrad]

I think the "Update Doxyfile" step should be between steps 1 and 2 of the release process.
Updating it to the next version in develop is premature IMO.

[oxarbitrage]

added Update Doxyfile to step 2 of the release process. Closing this one.