[Docs RFC] Update the contribution guide

[atovpeko]

1. Merge the following docs:

• https://github.com/timescale/docs/blob/latest/about/contribute-to-docs.md
• https://github.com/timescale/docs/blob/latest/README.md
• https://docs.timescale.com/about/latest/contribute-to-timescale/ (docs part)

2. Lean down, edit for readability.
3. Emphasize that a PR makes a working version of the docs so no adjustments to the build are necessary,
4. Put under https://docs.timescale.com/about/latest/contribute-to-timescale/ after the contributing to code part.
5. Remove https://github.com/timescale/docs/blob/latest/about/contribute-to-docs.md (?)