Update symlinks and remove md files from root (#609)

[lukasbriza]

Remove RELEASING.md and CONTRIBUTING.md and updating corresponding symlinks in docs/*.

[mbohal]

I think we should also do the same thing for the TESTING.md file. Presently it has the same content as src/docs/contribute/testing-guidelines.md, so I think we can:

1. remove TESTING.md
2. provide a link to src/docs/contribute/testing-guidelines.md from README.md

[lukasbriza]

I think we should also do the same thing for the TESTING.md file. Presently it has the same content as src/docs/contribute/testing-guidelines.md, so I think we can:

1. remove `TESTING.md`

2. provide a link to `src/docs/contribute/testing-guidelines.md` from `README.md`

Added testing-guidelines.md into docs and deleting TESTING.md

[lukasbriza]

@mbohal Should all links referencing parts of our documentation be internal links? I added links to the testing and releasing guidelines as external links, but for the testing link specifically, there’s no testing-guidelines.md file in master, so the GitHub check will fail due to a dead link. I'm wondering if it wouldn't be better to have all such references as internal links for consistency?

[mbohal]

I'm wondering if it wouldn't be better to have all such references as internal links for consistency?

I think we want this. We need links to work in IDE and if relevant also in the mkdocs output.
I think we need to use the relative syntax (../../abc) for this.

[lukasbriza]

After yesterday's call with @bedrich-schindler , it was decided to merge this task to enable further work on documenting the rules for writing Playwright tests.