docs: Adjust documents to reflect the new monorepo setup

[d3xter666]

JIRA: CPOUI5FOUNDATION-975

[maxreichmann]

1. The legacy branch name here (which results in a dead link) needs to be replaced with "v3" here:
   

   cli/docs/pages/extensibility/CustomTasks.md

   Line 205 in 7fd601d

   *For more details, see also [RFC 0012 UI5 CLI Extension API v3](https://github.com/UI5/cli/blob/rfc-ui5-tooling-extension-api-v3/rfcs/0012-UI5-Tooling-Extension-API-3.md#3-tasks-requiring-dependencies)*

2. There are some changelogs of the packages which contain outdated links e.g. here:

   cli/packages/builder/CHANGELOG.md

   Line 105 in 7fd601d

   Relates to: https://github.com/SAP/ui5-tooling/issues/701

   Due to our link forwarding, this might not be necessary :)

[d3xter666]

1. The legacy branch name here (which results in a dead link) needs to be replaced with "v3" here:
   

   cli/docs/pages/extensibility/CustomTasks.md

   Line 205 in 7fd601d

   *For more details, see also [RFC 0012 UI5 CLI Extension API v3](https://github.com/UI5/cli/blob/rfc-ui5-tooling-extension-api-v3/rfcs/0012-UI5-Tooling-Extension-API-3.md#3-tasks-requiring-dependencies)*

2. There are some changelogs of the packages which contain outdated links e.g. here:

   cli/packages/builder/CHANGELOG.md

   Line 105 in 7fd601d

   Relates to: https://github.com/SAP/ui5-tooling/issues/701

   
   Due to our link forwarding, this might not be necessary :)

I have updated the link in #1, but the second one is actually expected. There's a forwarding enabled in the old repo

[maxreichmann]

I have updated the link in #1, but the second one is actually expected. There's a forwarding enabled in the old repo

Although, we have this link forwarding, I think we should replace legacy links nevertheless. This might create confusion or even dead links if the forwarding is not working anymore at some point.
Perhaps, @flovogt knows more about this?

[flovogt]

I have updated the link in #1, but the second one is actually expected. There's a forwarding enabled in the old repo

Although, we have this link forwarding, I think we should replace legacy links nevertheless. This might create confusion or even dead links if the forwarding is not working anymore at some point. Perhaps, @flovogt knows more about this?

Forwarding will work as long as we do not stop it. So we don't have to worry about this.