Add integrations developer guide

[jonatas]

Description

I just created this for our first integration with Typescript. Let's make it public, and it will also be useful for further integrations.

Writing help

For information about style and word usage, see the style guide

Review checklists

Reviewers: use this section to ensure you have checked everything before approving this PR:

Subject matter expert (SME) review checklist

• Is the content technically accurate?
• Is the content complete?
• Is the content presented in a logical order?
• Does the content use appropriate names for features and products?
• Does the content provide relevant links to further information?

Documentation team review checklist

• Is the content free from typos?
• Does the content use plain English?
• Does the content contain clear sections for concepts, tasks, and references?
• Have any images been uploaded to the correct location, and are resolvable?
• If the page index was updated, are redirects required
  and have they been implemented?
• Have you checked the built version of this content?

[github-actions]

Allow 10 minutes from last push for the staging site to build. If the link doesn't work, try using incognito mode instead. For internal reviewers, check web-documentation repo actions for staging build status. Link to build for this PR: http://docs-dev.timescale.com/docs-add-integrators