Consider moving coding documentation elsewhere

[jpmckinney]

We have a page on coding, for lack of anywhere else to put it:

• https://ocds-standard-development-handbook.readthedocs.io/en/latest/coding/index.html

We also have a couple pages about GitHub and account management:

• https://ocds-standard-development-handbook.readthedocs.io/en/latest/systems/index.html

It's not obvious to look for them here. We can consider a better place to put them.

[jpmckinney]

See also https://crm.open-contracting.org/issues/4473 for discussion.

[jpmckinney]

Moved to https://ocp-software-handbook.readthedocs.io/en/latest/