Improve readability of Frameworks list + intro

[igorsantos07]

• added intro explaining about maintenance, official and deprecated libs
• removed redundant descriptions like "elm-i18next: use i18next in Elm" or repetitive and simple mentions to i18next (that's what this docs is for, right?
• fixed capitalization and naming of frameworks (many don't really use the .js suffix)
• improved some other descriptions so they're easier to read
• added mentions to original maintainers where repeated package naming exists, as well as pointing some packages that got deprecated or replaced

Closes #166

Checklist (for documentation change)

• [x] only relevant documentation part is changed
• [x] motivation/reason is provided (#166)

[adrai]

I don't know if the official label is appropriate... i.e. the ios and android packages were not created or maintained by the original creators of the i18next core...

[jamuhl]

being in the i18next org does not relate to being official or not -> I would leave that information as it makes no difference (just because it's from core maintainers does not grant any bonus over other OSS modules). Deprecated modules could be removed I guess - at least if we all agree they provide no huge benefit for devs.

[igorsantos07]

Gotcha!

The deprecated ones are still valid if someone for some reason prefers to use them. I didn't check features or anything else, so they might be useful, somehow. Not to mention three of these are the only plugins for some environments (Inferno / Polymer / Metalsmith). Do you think I should drop the ones that have "replacements" for the same env?

[adrai]

Do you think I should drop the ones that have "replacements" for the same env?

Yes, I think it's ok to remove those.

[igorsantos07]

Issues fixed :)

[adrai]

lgtm