Convert GitHub related documents to Markdown (#2051)

and remove FMI_CCLA.adoc

Author: t-sommer

CONTRIBUTING.adoc

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+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="docs/images/FMI-logo-horizontal-dark.svg">
+  <source media="(prefers-color-scheme: light)" srcset="docs/images/FMI-logo-horizontal-light.svg">
+  <img alt="Functional Mock-up Interface" src="docs/images/FMI-logo-horizontal-light.svg">
+</picture>
+
+# Functional Mock-up Interface
+
+The Functional Mock-up Interface (FMI) is a free standard that defines an interface to exchange dynamic models using a combination of XML files, binaries and C code.
+It's supported by [240+ tools](https://fmi-standard.org/tools/) and maintained as a [Modelica Association Project](https://modelica.org/projects).
+
+Releases and the [latest development version](https://fmi-standard.org/docs/main/) of the specification are available on the [FMI website](https://fmi-standard.org/).
+
+## Repository Structure
+
+- [docs](docs) &mdash; sources of the specification document
+- [headers](headers) &mdash; header files of the C-API
+- [schema](schema) &mdash; XSD schema for the modelDescription.xml
+- [CONTRIBUTING.md](CONTRIBUTING.md) &mdash; info on contributing to the project and building the specification document
+- [README.md](README.md) &mdash; this document
+
+## Branches
+
+- `main` &mdash; development of the next release
+- `v{2|3}.0.x` &mdash; maintenance of the 2.0 and 3.0 releases
+
+## Layered Standards
+
+The following [layered standards](https://github.com/modelica/fmi-standard/blob/main/docs/2_6_versioning_layered_standards.adoc#versioning-and-layered-standards) are currently under development:
+
+- [XCP](https://github.com/modelica/fmi-ls-xcp) &mdash; information for calibration and measurement
+- [Network Communication](https://github.com/modelica/fmi-ls-bus) &mdash; mapping of variables to buses
+- [Structure](https://github.com/modelica/fmi-ls-struct) &mdash; semantic grouping of variables
+- [REF](https://github.com/modelica/fmi-ls-ref) &mdash; stimuli and reference results
+
+## Copyright and License
+
+Code and documentation copyright &copy; 2008-2011 the MODELISAR consortium and 2012-2025 the Modelica Association Project FMI.
+Code released under the [2-Clause BSD License](https://opensource.org/licenses/BSD-2-Clause).
+Docs released under [Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/).

CONTRIBUTING.md

@@ -13,6 +13,6 @@ The licenses text can be found in the https://raw.githubusercontent.com/modelica
 
 Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights.
 Modelica Association shall not be held responsible for identifying such patent rights.
-All contributors to this specification have signed the https://github.com/modelica/fmi-standard/blob/master/FMI_CCLA.adoc[Corporate Contributor License Agreement of the FMI Project] or the https://github.com/modelica/ModelicaAssociationCLA/releases/download/1.1.1/ModelicaAssociationCLA_1.1.1.pdf[Contributor License Agreement of the Modelica Association].
+All contributors to this specification have signed the https://fmi-standard.org/FMI_CCLA_v1.0_2016_06_21.pdf[Corporate Contributor License Agreement of the FMI Project] or the https://github.com/modelica/ModelicaAssociationCLA/releases/download/1.1.1/ModelicaAssociationCLA_1.1.1.pdf[Contributor License Agreement of the Modelica Association].
 
 {empty}

FMI_CCLA.adoc

@@ -99,4 +99,4 @@
     });
 </script>
 
-<img style="display: block; margin: 2em auto; width: 30em;" src="images/FMI_logo_horizontal.svg" alt="FMI logo">
+<img style="display: block; margin: 2em auto; width: 30em;" src="images/FMI-logo-horizontal-light.svg" alt="FMI logo">