Improve handling of the multiple docs targets

[dmytro-kashcheiev]

In the current implementation, the following approach is used to handle multiple documentation targets:

code-path: path-to-code#1
docs-path: path-to-docs#1
other-options: some-values
embed-mappings:
  - code-path: path-to-code#2
    docs-path: path-to-docs#2

This solution is not obvious, since the embed-mappings field is a subfield of the root configuration and can override it, while its scope is effectively the same as the root. It is also error-prone, as parent settings can be overridden in sub-settings. The configuration should probably be defined in one of the following ways:

1. Single config or list of configs

For single use:

code-path: path-to-code#1
docs-path: path-to-docs#1

For multiple use:

embeddings:
  - name: some-name#1
    code-path: path-to-code#1
    docs-path: path-to-docs#1
    other-options: some-values
  - name: some-name#2
    code-path: path-to-code#2
    docs-path: path-to-docs#2
    other-options: some-values

2. List of embeddings in all cases

For single use:

embeddings:
  - name: some-name#1
    code-path: path-to-code#1
    docs-path: path-to-docs#1

For multiple use: same as above.