Update README with build environment notes

[AdmiralSnyder]

Added notes to Microsoft.AspNetCore.Components Readme.md

Added notes about environment prep and handling submodules for building from a git version archive.

Description

I tried to build the Microsoft.AspNetCore.Components project from the git version archive, and found a couple of points that imo should have been in the readme from the start.
see #68273 for details.

[dotnet-policy-service]

Thanks for your PR, @AdmiralSnyder. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[AdmiralSnyder]

@Youssef1313 pinging you since this is a conclusion from our discussion in #68273

[Copilot]

Pull request overview

Updates the Blazor Components area README to add additional guidance for building from source, especially around environment preparation and scenarios where submodules may be missing (e.g., source archives).

Changes:

• Adds an extra “Development Setup” note pointing readers to Build From Source environment preparation.
• Adds guidance about missing src/submodules content when building from a GitHub source archive.
• Adds a note after ./src/Components/build.cmd about potential E2E/Selenium-related failures and expected build output.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

Suppressed comments (3)

src/Components/README.md:33

• The new note links to the main branch docs on GitHub, which can diverge from the checked-out sources (especially when building from a release tag/source archive). Use the in-repo relative link so the documentation stays version-aligned.

**Note**: You also need to prepare your environment according to the steps in [`Build From Source`](https://github.com/dotnet/aspnetcore/blob/main/docs/BuildFromSource.md).

src/Components/README.md:54

• This note recommends manually downloading src/submodules from a GitHub tree URL (with a placeholder tag) if submodules are empty. GitHub source archives (zip/tar) don't include submodule contents, so the reliable guidance is to use a git clone with submodules (or run git submodule update --init --recursive), as described in docs/BuildFromSource.md. Also, the current text is hard to follow (capitalization/sentences) and contradicts the preceding “no Internet connection” statement.

   **Note**: if you are building from a github version archive, the submodules folders in src/submodules might be empty. you have to get them manually from   https://github.com/dotnet/aspnetcore/tree/vPUTTHEVERSIONHERE/src/submodules if `git submodule update --init --recursive` is not successful.

src/Components/README.md:83

• This note is inaccurate/misleading:

• ./src/Components/build.cmd doesn't run Selenium E2E tests; it invokes eng/build.cmd to build projects.
• It does force EnforceE2ETestPrerequisites=true, which runs the EnsurePrerequisites build target and can fail the build if prerequisites (e.g., Java on PATH) are missing.
• The artifacts path shouldn’t hardcode net10.0 (the repo’s DefaultNetCoreTargetFramework is currently net11.0).

     **Note**: This will also try to run some End-to-end tests using selenium which might fail. the artifact in `artifacts\bin\Microsoft.AspNetCore.Components\Debug\net10.0\Microsoft.AspNetCore.Components.dll` should have been built nonetheless.

[ilonatommy]

After fixing the updates copilot flagged I think it's ok to merge.