* Add docs about database migration

* Reorder upgrade from Gogs steps

* Update "Upgrade from Gogs" en-us doc

- Move "Change gogs specific information" FROM/TO steps up
  to just beneath matching section header, adjust indention
  and explicitly specify syntax of code sample to provide
  highlighting

- Adjust header levels to match what appears to be the
  intended level, remove trailing decoration (see below)

- Move "Upgrading to most recent ..." section down, remove
  decoration (section header formatting likely covers this
  well enough already)

refs go-gitea/gitea#4286, go-gitea/gitea#3558

* Update "Upgrade from Gogs" fr-fr doc

- Adjust indention and explicitly specify syntax of code
  sample to provide highlighting

- Adjust header levels to match what appears to be the
  intended level, remove trailing decoration since
  section header formatting likely covers this
  well enough already

refs go-gitea/gitea#4286, go-gitea/gitea#3558

Updates the manual for latest gogs changes.

* Clean up spelling, grammar, perspective, whitespace, language, markup, etc.

original PR go-gitea/docs#147 and author @0xbaadf00d

* import docs into main repository

Signed-off-by: Matti Ranta <matti@mdranta.net>