Issues and Pull Requests were not migrated to the new
organization/repository, so `matrix-org/synapse/pull` and
`matrix-org/synapse/issues` references were kept as-is.

`matrix-org/synapse-s3-storage-provider` references were also kept,
as that module still continues living under the `matrix-org` organization.

This patch mainly aims to change documentation-related things, not actual
usage in full yet. For polish that, another more comprehensive patch is coming later.

* added dendrite captcha options

* added hcaptcha doc

* proper url

* Apply suggestions from code review

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

* Update main.yml

* renamed captcha vars to new naming scheme

* change vars to new format

* Rename back some incorrect renamed variables

These variables are either not just part of the `client_api` subsection,
or are not even part of that section at all. They shouldn't have been
renamed in baaef2ed616e2645550d9

* Fix up naming inconsistencies

Some of these variables had been renamed in one place,
but not in other places, so it couldn't have worked that way.

* Add validation/deprecation for renamed Dendrite variables

Related to 4097898f885cf4c73, baaef2ed616e2645550, 68f4418092fa8ad
and a0b4a0ae6b2f1f18

Co-authored-by: Slavi Pantaleev <slavi@devture.com>