Rename README.md files in file tree to index.md

[BoHristova]

Why do we need the PR:
README.md files in the file tree retain their file name. When anchor links are resolved, README.md is converted to readme and inserted before the anchor, which breaks the link (e.g., https://gardener.cloud/docs/proposals/0001-gardener-extensibility/readme/#table-of-contents). As a result, all table-of-contents links in the Proposals section on gardener.cloud are broken (see gardener/documentation#857).

What does the PR do:
Updates manifest.go so that README.md files in the file tree are renamed to index.md.

Which issue(s) this PR fixes:
Fixes gardener/documentation#857

Special notes for your reviewer:

Release note:

NONE

[gardener-prow]

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[marc1404]

Have you tested this change with a development build of docforge in the documentation? It's difficult to judge the implications for all the places where README.md files are being pulled in.

So far, I haven't understood why Docforge turns relative anchor links into absolute links. Or more specifically, relative links that only refer to the same document, such as:
https://github.com/gardener/enhancements/blob/a9de75723e93627fd2eb8ae80bd3f1be29e3f9ff/geps/0001-gardener-extensibility/README.md#L5

- [GEP-0001: Gardener Extensibility and Extraction of Cloud-Specific/OS-Specific Knowledge (#308, #262)](#gep-0001-gardener-extensibility-and-extraction-of-cloud-specificos-specific-knowledge-308-262)

The easiest solution would be to keep these in-document anchor links exactly as given.

[BoHristova]

Have you tested this change with a development build of docforge in the documentation? It's difficult to judge the implications for all the places where README.md files are being pulled in.

Yes, I tested it with a development build of docforge in the Gardener documentation. But I understand your concerns, and I'm open to suggestions on how to fix the problem in a more straighforward way :)