Fix contributions link in API bindings docs

Closes #286
Fix link to dev blog in contributing docs

Closes #287
Replace run-on sentence with two seperate ones

As suggested in 4e0fdf8.
Change examples to za-peoples-assembly

Replace kenyan-politicians examples with za-peoples-assembly ones.

I also removed the note on the URL potentially 404ing if the data has reloaded, as that seemed more of an issue for auto-generated IDs than pre-constructed ones.
The API now returns membership information

Update the example JSON responses to show the membership information that is now included.
Remove quoting of slashes

Some links had needless \/s
Replace links to components list

Point at the Poplus mailing list rather than the old Components list.
Improve data-import documentation

* Fix broken link to /migration (fixes #317).
* Add link to the API docs.
* Small text tweaks.
Remove link to (obsolete) dev-blog

As part of documentation clean-up (Issue #322), remove the link to the dev-blog, which hasn't had a post in about a year.
Emphasise API usage over local installs

We ideally want people to use our hosted version of PopIt rather than
install it themselves. This change removes references to installing from
the main documentation site and emphasises using the hosted version.

Fixes #223
Update API homepage docs
Remove any documentation that is wrong/invalid

Changes to the documentation that were discussed with Tony.
More documentation updates

- Remove schemas page (since it only referred to images) and promote
  popolo link to the API overview page.
- Add a note about the za-peoples-assembly instance not using the posts
  collection.
- In the methods table make the unimplemented ones visually distinct.
Merge pull request #289 from mysociety/documentation-fixes

Documentation fixes